Dragonite-with-Steel (Or indeed any other Dragon/Flying Pokemon) added isn't vulnerable to Ground. It's immune. In fact, it's only weakness is Ice, which I've already said.
Meanwhile I said Fighting and Dark have reason to run Steel moves. Tyranitar, Scrafty, and Gyarados Mega (All of which see good use, particularly Tyranitar) are Dark types with STAB-less Iron Heads. Hydreigon is a Dark type that can run Flash Cannon and has cause to do so, what with being absolutely terrified of Fairies.
Since you brought up Iron Tail, let's look at Dark types that can run it: Absol, Houndoom, Krookodile, Liepard, Mightyena, Skuntank, Umbreon, Weavile, Drapion, and again Mega Gyarados, Hydreigon, and Tyranitar.
Heck, Bug is resisted by Fairy. (Though Scolipede is the only Bug that has a halfway decent Steel move without having STAB)
And, again, Steel is the most reliable counter to Fairy, full stop. If you are trying to make a Fairy counter, or Fairy check, for your team, Steel is the go-to move type.
Ground types adding Water to become resistant to Steel is also pretty irrelevant when they have to be part Steel (Steelix) or Fairy (Nothing) to resist-or-better Dragon moves. There's no point to becoming resistant to a coverage move while leaving yourself neutral to their primary attacking STAB.
Nor is a double weakness to Steel only a problem when facing any of these Pokemon. Any Pokemon on the enemy team that has a Steel move is a huge threat to you if you have a double Steel weakness. Being doubly weak to Steel when facing something you're supposed to counter that has access to Steel is especially problematic, but having a double weakness at all is dangerous, making it harder to switch in safely and wreaking havoc on your ability to wall, set-up, or heal if those are your goals. The only kind of Pokemon that largely doesn't care about a double weakness per se is one that's an all-or-nothing glass cannon that doesn't expect to survive being hit on a weakness at all, and is preferably running a Focus Sash anyway.
There are definitely differences in what types are hugely problematic vs mildly problematic to be doubly weak to. Stealth Rock means a double Rock weakness is uniquely awful, a double weakness to any of Fire, Fighting, or Ground is bad since those are the standard Steel answers people are usually running to counter Steel, while a double Poison weakness is sort of ignorable most of the time because Poison is rarely run as coverage and even Poison types themselves don't necessarily have an actual Poison attacking move if their goal is to just wear you down. But Steel is not a minor, ignorable thing to be doubly weak to.
I'm also baffled as to why you picked out Salamence and the Latis as examples of Dragons with no Steel moves directly after talking about bulky Ground types countering Dragon-with-Steel. Bulky Ground types countering Pokemon immune to their primary STAB? What? There isn't even a bulky Ground type with Mold Breaker out there to make the Latis afraid!
Also, this has taken entirely too long, but I finally wrapped up a major project.
Pokemon with Double Weaknesses
I'm going to divide this into two categories: Pokemon that can cover their double weakness with an immunity via type, and Pokemon that cannot, because there is an important difference in predictability there: Dragonite will always be weak to Ice, the only question is how weak it is. Whereas Toxicroak can just grab Dark to turn an attempt to Psychic it into a waste of a turn.
I'll be skipping relevant Fairies, since I've already covered them all in detail.
I'll also be focusing chiefly on those type additions that reduce or eliminate the double weakness in specific. Other type combinations may prove to be viable on a given Pokemon, but it's easier to focus on fixing the obvious problem, and can be enlightening in other regards.
It wouldn't surprise me if I missed some double weakness combo or another, but I'm pretty sure this is everything.
Freeze Dry might actually become important in this meta, depending on what Pokemon and specifically what types they prefer to add end up rising to prominence. So Aurorus, Articuno, Cryogonal, Delibird, Lapras, Mamoswine and Vanilluxe might rise in prominence for that alone.
Machamp, Toxicroak, Hariyama, Hitmonchan, Hitmonlee, and Hitmontop all have Bullet Punch without having STAB on it.
Meanwhile I said Fighting and Dark have reason to run Steel moves. Tyranitar, Scrafty, and Gyarados Mega (All of which see good use, particularly Tyranitar) are Dark types with STAB-less Iron Heads. Hydreigon is a Dark type that can run Flash Cannon and has cause to do so, what with being absolutely terrified of Fairies.
Since you brought up Iron Tail, let's look at Dark types that can run it: Absol, Houndoom, Krookodile, Liepard, Mightyena, Skuntank, Umbreon, Weavile, Drapion, and again Mega Gyarados, Hydreigon, and Tyranitar.
Heck, Bug is resisted by Fairy. (Though Scolipede is the only Bug that has a halfway decent Steel move without having STAB)
And, again, Steel is the most reliable counter to Fairy, full stop. If you are trying to make a Fairy counter, or Fairy check, for your team, Steel is the go-to move type.
Ground types adding Water to become resistant to Steel is also pretty irrelevant when they have to be part Steel (Steelix) or Fairy (Nothing) to resist-or-better Dragon moves. There's no point to becoming resistant to a coverage move while leaving yourself neutral to their primary attacking STAB.
Nor is a double weakness to Steel only a problem when facing any of these Pokemon. Any Pokemon on the enemy team that has a Steel move is a huge threat to you if you have a double Steel weakness. Being doubly weak to Steel when facing something you're supposed to counter that has access to Steel is especially problematic, but having a double weakness at all is dangerous, making it harder to switch in safely and wreaking havoc on your ability to wall, set-up, or heal if those are your goals. The only kind of Pokemon that largely doesn't care about a double weakness per se is one that's an all-or-nothing glass cannon that doesn't expect to survive being hit on a weakness at all, and is preferably running a Focus Sash anyway.
There are definitely differences in what types are hugely problematic vs mildly problematic to be doubly weak to. Stealth Rock means a double Rock weakness is uniquely awful, a double weakness to any of Fire, Fighting, or Ground is bad since those are the standard Steel answers people are usually running to counter Steel, while a double Poison weakness is sort of ignorable most of the time because Poison is rarely run as coverage and even Poison types themselves don't necessarily have an actual Poison attacking move if their goal is to just wear you down. But Steel is not a minor, ignorable thing to be doubly weak to.
I'm also baffled as to why you picked out Salamence and the Latis as examples of Dragons with no Steel moves directly after talking about bulky Ground types countering Dragon-with-Steel. Bulky Ground types countering Pokemon immune to their primary STAB? What? There isn't even a bulky Ground type with Mold Breaker out there to make the Latis afraid!
Also, this has taken entirely too long, but I finally wrapped up a major project.
Pokemon with Double Weaknesses
I'm going to divide this into two categories: Pokemon that can cover their double weakness with an immunity via type, and Pokemon that cannot, because there is an important difference in predictability there: Dragonite will always be weak to Ice, the only question is how weak it is. Whereas Toxicroak can just grab Dark to turn an attempt to Psychic it into a waste of a turn.
I'll be skipping relevant Fairies, since I've already covered them all in detail.
I'll also be focusing chiefly on those type additions that reduce or eliminate the double weakness in specific. Other type combinations may prove to be viable on a given Pokemon, but it's easier to focus on fixing the obvious problem, and can be enlightening in other regards.
Toxicroak: Adding Dark is of course fantastic and an excellent fit to Toxcroak's movepool, removing its primary weakness entirely while only adding a weakness to Fairy, which is less bad than it sounds when Toxicroak outspeeds a lot of Fairies, has STAB Poison Jab (Or Gunk Shot!), and would rather switch out when faced with the likes of Togekiss anyway. Adding Steel would provide it with a double weakness to Ground, and also synergizes poorly with Dry Skin increasing Fire damage taken in the first place, even though it reduces the Psychic weakness and removes the Flying weakness. Adding Psychic only adds a Ghost weakness, but it also only reduces the Psychic weakness one step. Alternatively, one could potentially focus on abusing Dry Skin: Toxicroak does learn Earthquake, so adding Ground could be good, and with Dry Skin it would only add an Ice weakness. Or you could add Rock, adding a weakness to Steel and giving it a double weakness to Ground, and run Stone Edge. (Eugh) Adding Fire offers it Burn immunity (And mitigates Dry Skin's added Fire vulnerability), but gives it, again, a double weakness to Ground, while still leaving it with a double weakness to Psychic, hooray. Overall, I'd either add Dark or Ground.
Gyarados (not Mega Gyarados): Adding Ground of course eliminates the Electric weakness outright, and also by the way eliminates the Rock weakness, leaving it with weaknesses to Grass and Ice. (Also it removes the Stealth Rock weakness) Oh, and you take x8 damage from Freeze Dry, so beware anything with Freeze Dry. STAB Earthquake too! Alternatively, you could add Electric, reducing the Electric weakness by one step at no added cost, and without the x8 damage from Freeze Dry. (It's just x4) Or you could add Grass, reducing the Electric weakness by one step, adding in vulnerability to Poison, Ice, Bug, and Flying, still taking x8 damage from Freeze Dry, and just generally hating yourself because you didn't add Ground instead. Lastly, you can add Dragon, which reduces the Electric weakness, adds Ice, Fairy, and Dragon weaknesses, and places you at x8 damage from Freeze Dry again. (Just take Ground and hate yourself less) If for some reason you'd rather protect yourself from Freeze Dry and ignore the Electric weakness, you can add Ice, Fire, or Steel. The first two both leave you doubly weak to Rock (Stealth Rock), while the latter actually has no added weakness and even removes the Rock weakness! It might be worth adding Steel to Gyarados after all. There's probably STAB options worth looking into, but really, adding Ground is probably Gyarados' best thing to do for several reasons, even with the horrible Freeze Dry vulnerability.
Mantine: Everything I said about Gyarados still applies, except Mantine can't really appreciate the Ground STAB. (Even though it gets Earthquake, its Attack is a mere 40, and it gets no good Special Ground moves) It's probably still worth taking Ground in specific, just because Mantine really wants to wall. Plus, it can use Water Absorb to ignore now being neutral to Water, so that's nice.
Pelipper: I don't think there's really anything to add, except that it would really appreciate being able to meaningfully wall and actually use its Rain Dish Ability.
Swanna: Eeeeh whatever.
Heatran: As has been shown off by The Immortal, Flying is a fantastic combination for Heatran, not only wiping its Ground weakness but also covering up its Fighting weakness, but in trade Heatran picks up weaknesses to Electric and Rock. Adding Bug to it would reduce its Ground weakness to one tier, also eliminate its Fighting weakness, and the only loss for most purposes is a vulnerability to Rock. (Beware Mold Breakers with Fire moves, though) Adding Grass instead would reduce the Ground weakness, eliminate the Water weakness, and add no new weakness barring Mold Breaker Fire attackers. Arguments can be made for all three options, actually, and Grass in particular is a good way of punishing opponents who simply assume your Heatran is part-Flying, since you'll outright resist Electric, not to mention eliminating the Water weakness is nice. Plus in a Sun team it's adding STAB to Solar Beam! It's hard to make an argument for other types: Heatran's options to add STAB to generally make its Ground weakness even worse and/or make its Water weakness worse, or in the case of Dark Pulse make its Fighting weakness worse. Dragon Pulse is the only real exception, covering up its Water weakness, giving it Electric resistance, and opening no new weaknesses, but you still have a double weakness to Ground. A weak argument can also be made for adding Ghost, to punish High Jump Kick Pokemon basically. Overall, you're probably best off sticking to Flying, Grass, or Bug, whichever one you think works best.
Magnezone: Same basic situation as Heatran, except Electric has fewer weaknesses but it isn't Fire: adding Flying to it results in a Pokemon whose only weakness is Fire. Adding Grass or Bug to it on the other hand has a higher cost than on Heatran, leaving it doubly weak to Fire, while having less benefit than adding Flying. In short: Magnezone should probably only ever add Flying, unless you're specifically trying to take advantage of the opponent assuming that's what you took, in which case Water is probably the way to go, adding no weaknesses and covering up the Fire vulnerability that they'll expect to be able to pick on.
Aggron: Aggron has the unfortunate situation that you can completely eliminate both of its double weaknesses, but not at the same time. If you add Ghost, it eliminates the double Fighting weakness in exchange for Ghost and Dark vulnerability. If you add Flying, it completely eliminates the Ground weakness, reduces the Fighting weakness one step, and adds a weakness to Electric. (OK, so my opening sentence was slightly misleading) Alternatively, you could add Bug, adding a Fire weakness but reducing both the Fighting and Ground weaknesses to one step apiece. (Overall worse than just adding Flying) Or you could add Grass to reduce the Ground weakness one step, add a Fire weakness, have no effect on the double Fighting weakness. (dubious) Adding Poison to reduce the Fighting weakness results in a triple Ground weakness, no thanks. Basically, just add Flying, full stop.
Bastiodon: What I said about Aggron.
Weavile: Adding Ghost removes the double Fighting weakness plus the Bug weakness and adds no new weaknesses, leaving it weak to Rock, Steel, Fire and Fairy. This is probably Weavile's best option, period. Nonetheless: if it adds Poison, it reduces the Fighting weakness, gains immunity to Toxic et all, and only gains a Ground weakness in turn. If it adds Bug, it is suddenly doubly weak to both Fire and Rock, don't do that. If it adds Psychic, that's a crappy Ghost that gives it a double weakness to Bug, don't do that. Adding Flying leaves it doubly weak to Rock, throws in a weakness to Electric, and only reduces the FIghting weakness and eliminates the Bug weakness, so a bad Ghost in short. And you can't add Fairy, which would give it a double weakness to Steel even if you could, so it would be dumb. Really, stick with Ghost unless you desperately want STAB on some move or another. I can't imagine what would be worth the double or even triple weaknesses that can result, though. (Add Normal for boost Fake Out, for instance, and now it's triply weak to Fighting, add Fighting for Low Kick STAB or something and it's doubly weak to Fairy in addition to Fighting, etc)
Tyranitar: Same basic thing as Weavile: add Ghost, remove the Fighting and Bug weakness, remain weak to Steel, Fairy, Ground, Grass, and Water. Poison gives it a double weakness to Ground, Bug actually isn't that terrible but isn't amazing either (Reduces Fighting and removes Grass and Ground as weaknesses, but adds Rock as a weakness), Flying's only upshot over Ghost is eliminating the Grass weakness, adding Psychic is still directly inferior to adding Ghost... just stick with Ghost, seriously.
Bisharp: Like Weavile and Tyranitar, adding Ghost eliminates the Fighting weakness outright with no added disadvantage, leaving it weak to Fire and Ground alone. Bug and Grass both give you a double weakness to Fire in exchange for removing the Ground weakness and in Bug's case reducing the Fighting weakness, Flying adds an Electric weakness in exchange for removing the Ground weakness and reducing the Fighting weakness, Psychic is just bad Ghost that adds in a Bug weakness... I'd really just recommend taking Ghost.
Gyarados (not Mega Gyarados): Adding Ground of course eliminates the Electric weakness outright, and also by the way eliminates the Rock weakness, leaving it with weaknesses to Grass and Ice. (Also it removes the Stealth Rock weakness) Oh, and you take x8 damage from Freeze Dry, so beware anything with Freeze Dry. STAB Earthquake too! Alternatively, you could add Electric, reducing the Electric weakness by one step at no added cost, and without the x8 damage from Freeze Dry. (It's just x4) Or you could add Grass, reducing the Electric weakness by one step, adding in vulnerability to Poison, Ice, Bug, and Flying, still taking x8 damage from Freeze Dry, and just generally hating yourself because you didn't add Ground instead. Lastly, you can add Dragon, which reduces the Electric weakness, adds Ice, Fairy, and Dragon weaknesses, and places you at x8 damage from Freeze Dry again. (Just take Ground and hate yourself less) If for some reason you'd rather protect yourself from Freeze Dry and ignore the Electric weakness, you can add Ice, Fire, or Steel. The first two both leave you doubly weak to Rock (Stealth Rock), while the latter actually has no added weakness and even removes the Rock weakness! It might be worth adding Steel to Gyarados after all. There's probably STAB options worth looking into, but really, adding Ground is probably Gyarados' best thing to do for several reasons, even with the horrible Freeze Dry vulnerability.
Mantine: Everything I said about Gyarados still applies, except Mantine can't really appreciate the Ground STAB. (Even though it gets Earthquake, its Attack is a mere 40, and it gets no good Special Ground moves) It's probably still worth taking Ground in specific, just because Mantine really wants to wall. Plus, it can use Water Absorb to ignore now being neutral to Water, so that's nice.
Pelipper: I don't think there's really anything to add, except that it would really appreciate being able to meaningfully wall and actually use its Rain Dish Ability.
Swanna: Eeeeh whatever.
Heatran: As has been shown off by The Immortal, Flying is a fantastic combination for Heatran, not only wiping its Ground weakness but also covering up its Fighting weakness, but in trade Heatran picks up weaknesses to Electric and Rock. Adding Bug to it would reduce its Ground weakness to one tier, also eliminate its Fighting weakness, and the only loss for most purposes is a vulnerability to Rock. (Beware Mold Breakers with Fire moves, though) Adding Grass instead would reduce the Ground weakness, eliminate the Water weakness, and add no new weakness barring Mold Breaker Fire attackers. Arguments can be made for all three options, actually, and Grass in particular is a good way of punishing opponents who simply assume your Heatran is part-Flying, since you'll outright resist Electric, not to mention eliminating the Water weakness is nice. Plus in a Sun team it's adding STAB to Solar Beam! It's hard to make an argument for other types: Heatran's options to add STAB to generally make its Ground weakness even worse and/or make its Water weakness worse, or in the case of Dark Pulse make its Fighting weakness worse. Dragon Pulse is the only real exception, covering up its Water weakness, giving it Electric resistance, and opening no new weaknesses, but you still have a double weakness to Ground. A weak argument can also be made for adding Ghost, to punish High Jump Kick Pokemon basically. Overall, you're probably best off sticking to Flying, Grass, or Bug, whichever one you think works best.
Magnezone: Same basic situation as Heatran, except Electric has fewer weaknesses but it isn't Fire: adding Flying to it results in a Pokemon whose only weakness is Fire. Adding Grass or Bug to it on the other hand has a higher cost than on Heatran, leaving it doubly weak to Fire, while having less benefit than adding Flying. In short: Magnezone should probably only ever add Flying, unless you're specifically trying to take advantage of the opponent assuming that's what you took, in which case Water is probably the way to go, adding no weaknesses and covering up the Fire vulnerability that they'll expect to be able to pick on.
Aggron: Aggron has the unfortunate situation that you can completely eliminate both of its double weaknesses, but not at the same time. If you add Ghost, it eliminates the double Fighting weakness in exchange for Ghost and Dark vulnerability. If you add Flying, it completely eliminates the Ground weakness, reduces the Fighting weakness one step, and adds a weakness to Electric. (OK, so my opening sentence was slightly misleading) Alternatively, you could add Bug, adding a Fire weakness but reducing both the Fighting and Ground weaknesses to one step apiece. (Overall worse than just adding Flying) Or you could add Grass to reduce the Ground weakness one step, add a Fire weakness, have no effect on the double Fighting weakness. (dubious) Adding Poison to reduce the Fighting weakness results in a triple Ground weakness, no thanks. Basically, just add Flying, full stop.
Bastiodon: What I said about Aggron.
Weavile: Adding Ghost removes the double Fighting weakness plus the Bug weakness and adds no new weaknesses, leaving it weak to Rock, Steel, Fire and Fairy. This is probably Weavile's best option, period. Nonetheless: if it adds Poison, it reduces the Fighting weakness, gains immunity to Toxic et all, and only gains a Ground weakness in turn. If it adds Bug, it is suddenly doubly weak to both Fire and Rock, don't do that. If it adds Psychic, that's a crappy Ghost that gives it a double weakness to Bug, don't do that. Adding Flying leaves it doubly weak to Rock, throws in a weakness to Electric, and only reduces the FIghting weakness and eliminates the Bug weakness, so a bad Ghost in short. And you can't add Fairy, which would give it a double weakness to Steel even if you could, so it would be dumb. Really, stick with Ghost unless you desperately want STAB on some move or another. I can't imagine what would be worth the double or even triple weaknesses that can result, though. (Add Normal for boost Fake Out, for instance, and now it's triply weak to Fighting, add Fighting for Low Kick STAB or something and it's doubly weak to Fairy in addition to Fighting, etc)
Tyranitar: Same basic thing as Weavile: add Ghost, remove the Fighting and Bug weakness, remain weak to Steel, Fairy, Ground, Grass, and Water. Poison gives it a double weakness to Ground, Bug actually isn't that terrible but isn't amazing either (Reduces Fighting and removes Grass and Ground as weaknesses, but adds Rock as a weakness), Flying's only upshot over Ghost is eliminating the Grass weakness, adding Psychic is still directly inferior to adding Ghost... just stick with Ghost, seriously.
Bisharp: Like Weavile and Tyranitar, adding Ghost eliminates the Fighting weakness outright with no added disadvantage, leaving it weak to Fire and Ground alone. Bug and Grass both give you a double weakness to Fire in exchange for removing the Ground weakness and in Bug's case reducing the Fighting weakness, Flying adds an Electric weakness in exchange for removing the Ground weakness and reducing the Fighting weakness, Psychic is just bad Ghost that adds in a Bug weakness... I'd really just recommend taking Ghost.
Dragon/Flying: I've already mentioned that adding Steel leaves Dragonite with Ice as its only weakness, which synergises nicely with the fact that it has Iron Head and likes to use it to counter Fairies. Other options include: add Ice, and cry at having a double Rock weakness plus a Steel weakness and the only payoff is reduced Ice damage and immunity to Freeze. Add Water, gaining an Electric weakness, becoming triply vulnerable to Freeze Dry, and cry. Add Fire, and cry at having another double Rock weakness. Or add some other type that still isn't Steel, and... um... cry a lot. You'd like STAB on Earthquake right? You don't mind a triple Ice weakness of course.
Seriously just add Steel, never look back. As for its buddies, Rayquaza, Salamence, Altaria, and Noivern? Same thing. Add Steel, never look back, don't care that they don't all have Steel moves like Dragonite. (Mostly this means they're bad Dragonites in this meta)
Dragon/Ground: Adding Steel to Ground/Dragon is a little different, adding both a Fighting and a Ground weakness while covering up the Fairy and Dragon weaknesses and reducing the Ice weaknesses. It's nice to not have a double weakness though? Adding Water adds a Grass weakness, places you at x8 damage from Freeze Dry (But hey you're only one stage vulnerable to regular Ice), and doesn't really do much else unless you really want a double Fire resistance or something. Adding Fire adds a Ground and Water weakness, eliminates the Fairy weakness in addition to reducing the Ice weakness, and provides immunity to being Burned, which is important to all three Ground/Dragon Pokemon. And of course Adding Ice is pretty sub-optimal, though it actually only needs a Steel and Fighting weakness here. Mostly Zygarde (And Flygon, and Garchomp) must look enviously upon Dragonite (And its buddies), wishing they could be as awesome. Flygon can actually learn Fire Punch, so it should give serious thought to adding Fire. Of the three, Flygon also has the advantage that it gets to ignore the Ground vulnerability, leaving the enemy to guess whether your Flygon is vulnerable to Fighting or Water if they don't have Ice to pick on it. (Though they really should have Ice, just because Dragonite is kind of ridiculous)
Grass/Flying: Adding Steel to Grass/Flying results in a double weakness to Fire (same for adding Ice), adding in Fire results in a double Rock weakness, adding in Water results in a double Electric weakness (And a triple Freeze Dry weakness)... Tropius, Jumpluff, and Shaymin-Sky cannot escape having a double weakness, and none of them have Abilities to soften the blow. Nonetheless, adding Fire or Water may be worth it just because Ice has priority in the form of Ice Shard, while Electric and Fire are currently incapable of priority, which can make a big difference for Shaymin-Sky and Jumpluff in particular, who normally rely on their speed to help protect them.
Torterra: Adding Steel is again a double weakness to Fire (And again Ice is too), but adding in Water actually adds no weaknesses while not only reducing the Ice weakness but eliminating the Fire weakness. (That triple weakness to Freeze Dry though) Adding Fire on the other hand reduces the Ice weakness, eliminates the Fire and Bug weaknesses, renders Torterra immune to Burns, and the only weakness it adds is to Water. (It also avoids Freeze Dry dooming you) Either one could be good for Torterra.
Ground/Flying: Adding Steel reduces the Ice weakness at no real cost. (Not so much adding Ice) Unfortunately, this combines poorly with Gliscor's desire to utilize Poison Heal, but Landorus is probably perfectly happy to take Steel. Adding Water reduces the Ice weakness and eliminates the Water weakness, but adds in a Grass weakness and brings into play the Freeze Dry=doom problem. Lastly, adding Fire reduces the Ice weakness, adds a Rock weakness (Including Stealth Rock!), and turns the Water weakness into a double weakness. But hooray for Burn immunity? (Gliscor would rather just take Poison Heal and add Water) Basically, Landorus should probably just take Steel, while Gliscor should probably just take Water and be ready to drop everything and flee from anything that can get Freeze Dry.
Fire/Flying: Adding Steel reduces the Rock weakness, with the only cost being if you want to Roost and something Earthquakes you or similar. Adding Fighting also reduces the Rock weakness, but adds in a Flying and Psychic weakness, though it doesn't add any situational double weaknesses either. Adding Ground adds an Ice weakness, doubles the Water weakness, and OK yeah reduces the Rock weakness. (Which is maybe a worthwhile trade if Stealth Rock is your big concern) Mostly, Charizard, Talonflame, Moltres, and Ho-oh should just add Steel and not look back. (Unless we're talking Mega Charizard X, which is a whole different thing) To be fair, Mega Charizard Y often runs Focus Blast anyway, so having STAB to really wreck Heatran would be nice... on the other hand, Heatran is generally going to be Flying anyway, so it won't actually be vulnerable!
Volcarona: Adding Steel adds a Ground and Fire weakness, but reduces the Rock weakness, and removes the Flying weakness. Decent trade. Adding Fighting reduces the Rock weakness, but gives it a double Flying weakness and adds in a Psychic weakness hooray don't do it. Adding Ground reduces the Rock weakness, doubles the Water weakness, and is just kinda not good. Basically, unless you want to just accept the double Rock weakness (And it's a huge part of what holds Volcarona back), tack on Steel, however tempting it might be to do something like add Grass for Giga Drain STAB. (Which results in another double Flying weakness, by the way)
Camerupt: Adding Water adds a Grass weakness, reduces the Water weakness, and that's about it. (But you're weak to Freeze Dry) Adding Grass reduces the Water weakness, removes the Ground weakness, and adds in a Flying weakness. Adding Dragon reduces the Water weakness, and adds a Dragon and Ice weakness, hooray. Grass is actually probably it's best option, being a net positive in effectives and throwing in immunity to Spore et al for free. This of course all applies to Simple Eviolite Numel too.
Magcargo: You can bypass its double weakness to Ground with Flying, actually, so one could argue it belongs in the first category, but its double weakness to Water cannot be erased with some other type. Now, there's good news: adding Grass reduces both its double weaknesses to single weaknesses, with no added weaknesses! (And throwing in immunity to Spore et al to boot) No other type resists both Water and Ground at the same time, but that's not so bad when adding Grass is so perfect. Nonetheless, for your edification: you can add Flying to be immune to Ground but go up to doubly weak to Rock and just no ugh, you can add Bug to reduce the Ground weakness but again become doubly weak to Rock and ugh, you can add Water to reduce its Water weakness but gain both Electric and Grass weaknesses, or you can add Dragon to pick up a Dragon weakness while reducing the Water weakness and it's actually kind of nice overall. But mostly: add Grass, put it on a Sun team (Now your Water weakness is truly neutered!) give it Solar Beam, be amazed at Magcargo actually not completely sucking!
Or probably still don't use it because it's still kind of mediocre and its Abilities in particular are sad.
Water/Rock: Adding Grass reduces the Grass weakness, removes the Ground and Electric weaknesses, in exchange adding a Bug weakness, and making Freeze Dry murder you. Adding Bug reduces the Grass weakness, removes the Ground and Fighting weaknesses, in exchange adds a Rock weakness, and is OK generally. Adding Steel results in double weakness to both Ground and Fighting why would you do this to yourself? Adding Poison results in another double Ground weakness while adding a Psychic weakness too, though to be fair it actually both reduces the Grass weakness and also eliminates the Fighting weakness, but even so. Adding Flying results in a double Electric weakness, hooray, and a double weakness to Freeze Dry, double hooray. Adding Dragon reduces the Grass weakness and eliminates the Electric weakness, throws in Fairy and Dragon weaknesses, and again makes Freeze Dry murder you. Anything else does nothing about the double Grass weakness. Overall, Barbaracle, Relicanth, Carracosta, Omastar, Kabutops, and Corsola are probably best off with either Bug or Grass.
Ground/Water: Adding Grass reduces the Grass weakness, adds in an Ice weakness (And thus raises Freeze Dry from x4 to x8 damage) plus Flying and Bug weaknesses, but hey immunity to Spore and all? Adding Bug reduces the Grass weakness, adds in a Flying weakness, and that's pretty OK. Adding Dragon reduces the Grass weakness while adding in Fairy, Ice, and Dragon weaknesses, which includes Freeze Dry murdering you, don't do it. Adding Poison reduces the Grass weakness while adding in both Psychic and Ground weaknesses, which is maybe worth it? Adding Steel reduces the Grass weakness while adding in Ground and Fighting weaknesses. Keep in mind it also reduces the Freeze Dry weakness to one tier. Adding Flying reduces the Grass weakness but adds an Ice weakness including of course increasing Freeze Dry's instagib potential. Overall, Swampert, Gastrodon and Seismitoad are probably best off adding Bug or Steel, or maybe Poison if you have too many Fighting weaknesses as is on the team. Gastrodon finds Grass even less appealing than usual, since the resistance to Water is provides is largely irrelevant when it's probably running Storm Drain anyway.
Grass/Psychic: Adding Ghost reduces the Bug weakness but redoubles the Dark and Ghost weaknesses. No. Adding Flying reduces the Bug weakness, but adds in a Rock weakness and doubles the Ice weakness. Adding in Poison reduces the Bug weakness, removes the Poison weakness, and adds no new weaknesses. Very nice, especially since it throws in Toxic immunity. Adding in Steel reduces the Bug weakness and eliminates the Ice weakness, but doubles the Fire weakness. Eeeeeh. You can't add Fairy, and it would be kind of terrible to add anyway. Adding Fighting reduces the Bug weakness and eliminates the Dark weakness, but doubles the Flying weakness and adds a Fairy weakness. Lastly, adding Fire reduces the Bug weakness, removes the Fire and Ice weakness, but adds in a Rock weakness. Overall Poison and Fire are, by far, the best things to add to Celebi or Exeggutor.
Malamar: Adding Ghost reduces the Bug weakness but adds in Dark and Ghost weaknesses. Adding Poison reduces the Bug weakness, removes the Fairy weakness, and only adds a Ground weakness in exchange. Quite nice. Adding in Fighting reduces the Bug weakness, doubles the Fairy weakness, and adds in a Flying weakness too. No. Adding in Fire reduces the Bug weakness, removes the Fairy weakness, and adds in Ground, Rock, and Water weaknesses. So a bad Poison. Adding in Flying reduces the Bug weakness, but adds in Rock, Electric, and Ice weaknesses. Awful. Still can't add Fairy, though it would actually be kind of OK if you could. Lastly, adding in Steel reduces the Bug weakness and removes the Fairy weakness, but adds in Fire, Ground, and Fighting weaknesses ie again a bad Poison, particularly since they both provide immunity to Toxic et al but Poison lets you absorb Toxic Spikes. If you're going to use Malamar, there's only really two routes: add Poison, or add Fighting and accept the double Fairy weakness in exchange for Contrary Superpower STAB.
Cacturne: Adding Ghost reduces the Bug weakness and eliminates the Fighting and Poison weaknesses with no added weaknesses. Nice. Adding Poison reduces the Bug weakness, removes the Fighting and Poison weakness, and also adds no new weaknesses. Adding Fighting reduces the Bug weakness, doubles the Fairy and Flying weaknesses... no. Adding Fire reduces the Bug weakness, eliminates the Fire and Ice weaknesses, and only adds a Rock weakness in exchange. Decent. Adding in Steel reduces the Bug weakness, removes the Ice and Poison weaknesses, but doubles the Fire and Fighting weaknesses. Ugh. Adding in Flying reduces the Bug weakness, eliminates the Fighting weakness, adds a Rock weakness and doubles the Ice weakness. Eugh. Overall Cacturne is probably best off with your choice of Ghost or Poison (Ghost is straight up immune to Fighting, but Poison cannot be Poisoned), or maybe Fire if it better suits your team or if Cacturne becomes popular enough people start assuming that it's Ghost or Poison and just hit it with Fire or Ice.
Aurorus: Like Magcargo, Aurorus can eliminate one of its double weaknesses, but not the other. Adding Ghost will of course completely eliminate its double Fighting weakness, but it has no effect on its double Steel weakness and adds Dark and Ghost weaknesses. There is in fact no single type that resists both Fighting and Steel: if you add Steel, Water, Electric, or Fire to reduce the double Steel weakness, the double Fighting weakness is unaffected, while adding Ghost, Flying, Bug, Poison, Psychic (Or Fairy, but you can't do that anyway) has no effect on the double Steel weakness. (Except Fairy would actually make it even worse) Bonus points: adding Water doubles the Grass weakness, adding Electric (Which it actually has STABs for) Poison or Fire doubles the Ground weakness as does Steel while also bring the Fighting weakness up to triple weakness, while adding Flying or Bug doubles the Rock weakness. Only Psychic and Ghost don't replace the double weakness they're clearing with at least one other, and of the two Ghost is basically flatly superior. In short, Aurorus is still screwed.
Bug/Flying: Alas, that there are only three types that resist Rock. Adding Fighting results in a double Flying weakness, adding Steel results in a double Fire weakness, and adding Ground results in a double Ice weakness. Of these, Steel (Adds no other weaknesses) and Ground (Also adds Water as a weakness, fewer resistances than Steel) are probably your best bet for Butterfree, Beautifly, Vespiquen, Mothim, Masquerain, Ninjask, Yanmega, and Vivillon. Or you could ignore the problem entirely, but a double weakness to Rock is problematic when that means Stealth Rock can 2HKO you on its own. In short: Bug/Flying is still an awful typing and you probably shouldn't use any of its representatives even in this meta without a really good reason. Mostly: add Steel, and plan around it. (Note that Vivillon's access to Powder can punish attempts to incinerate it, or psych people out into not using it at all)
Ice/Flying: Like the Bugs, but the news is a little better actually. Adding Steel will still result in a double Fire weakness, and also add in a Fighting weakness while its at it, but Fighting results in no double weaknesses (It still adds all of Psychic, Flying, and Fairy to your list), and Ground too results in no double weaknesses. (Adds Water and Ice weaknesses) Fighting and Ground being added are both viable for Articuno and Delibird. Now, whether either of them is viable is a whole other matter...
Bug/Steel: The only thing to deal with is a double Fire weakness, and I'm not going to repeatedly state its being reduced. Adding Water adds in an Electric weakness. Adding in Dragon adds no weaknesses at all. Adding in Fire adds a Ground, Rock, and Water weakness, don't do it. Adding in Rock adds a Fighting and Water weakness. In short, Forretress, Scizor, Escavalier, Wormadam-Trash, and Genesect should all add Dragon and never look back unless your specific goal is to acquire STAB on a move. (As if Forretress cares)
Ferrothorn: Adding Rock reduces the Fire weakness but makes the Fighting weakness worse. Adding Dragon reduces the Fire weakness but brings in an Ice weakness. Adding Water reduces the Fire weakness but adds a Freeze Dry weakness. Adding Fire reduces the Fire weakness but adds in Ground and Rock weaknesses. Overall Ferrothorn is probably best off adding Water in practice.
Abomasnow: Adding Rock reduces the Fire weakness, removes the Poison and Flying weaknesses, but doubles both the Steel and Fighting weaknesses. Eugh. Adding Dragon reduces the Fire weakness, adds in Ice, Dragon, and Fairy weaknesses, and that's it. Adding Water reduces the Fire weakness, removes the Steel weakness, and adds no new weaknesses. (Except Freeze Dry) Adding Fire reduces the Fire weakness, removes the Bug and Steel weaknesses, but doubles the Rock weakness. (Oh god Stealth Rock) Add Water, don't look back, basically? Unless you're personal goal is to remove the Stealth Rock weakness I guess.
Bug/Grass: With a double weakness to both Flying and Fire, this is torture. Fortunately, adding Rock reduces both of those and also removes the Poison weakness, with its only cost being adding in a Steel weakness, because Bug/Grass resists or double resists all of Rock's other weaknesses. Very nice. Alternatively, you can add Dragon (double Ice weakness, Fairy Weakness, Dragon weakness, only reduces Fire damage taken), Fire (Double Rock weakness, though it also removes the Bug weakness), or Water (No added weaknesses and removes the Ice weakness, barring Freeze Dry) to reduce the Fire weakness while doing nothing about the double Flying weakness, or you can add Electric (No added weakness) or Steel (Makes the Fire weakness even worse, though to be fair it also clears the Poison, Bug, Ice, and Rock weaknesses, leaving you with just a Flying weakness and a triple Fire weakness) to reduce the Flying weakness without addressing the double Fire weakness. The general best choice is Rock, but Steel might be the true best choice for Parasect, Wormadam-Grass and Leavanny, since it narrows them down to just two weaknesses. Just beware... you know... anything with so much as Ember (Especially Dry Skin Parasect) and you'd be surprisingly decent.
Grass/Fighting: Adding Rock reduces the Flying weakness, removes the Poison weakness, and adds in Fighting and Steel weaknesses. Adding Electric reduces the Flying weakness with no added weaknesses. Adding Steel reduces the Flying weakness, removes the Poison, Psychic, Fairy, and Ice weaknesses, but makes the Fire weakness a double weakness and adds a Fighting weakness. The general best choice for Breloom, Chesnaught and Virizion is probably to add Electric, but Steel can be a worthwhile trade-off and for Virizion it may be worth going for Rock to gain STAB on some of its only off-type moves. Keep in mind Chesnaught's Bulletproof can also let it shrug off some weaknesses more readily than others, though nothing stands out to me as strongly relevant unfortunately.
Ground/Rock: Double weakness to both Water and Grass hooray. Good news: adding Grass reduces both of these weakness at the same time! Bad news: it leads to a double Ice weakness and adds a Bug weakness. Adding Dragon also reduces both weaknesses and also results in a double Ice weakness while adding in Fairy and Dragon weaknesses. There's just no escaping a double weakness for Golem and Rhyperior. Even so... adding Water reduces the Water weakness, removes the Ice weakness, and leads to a triple Grass weakness and a double Freeze Dry weakness. Adding Flying reduces the Grass weakness, removes the Fighting and Ground weaknesses, doubles the Ice weakness yipee. Adding Bug reduces the Grass weakness and removes the Fighting weakness with no added disadvantage. Adding Fire reduces the Grass weakness, doubles the Ground weakness, leads to a triple Water weakness, and is just awful. Adding Poison reduces the Grass weakness and removes the Fighting weakness, but the Ground weakness is doubled and a Psychic weakness is added. Lastly, adding Steel reduces the Grass weakness, removes the Ice weakness, and leads to a double weakness to both Ground and Fighting. Ground/Rock has the dubious honor of being the first type combination I've found can give itself three double weaknesses by adding a type. Ugh.
Fighting/Dark: Adding Fire reduces the Fairy weakness, but adds in Ground and Water weaknesses. Adding Poison reduces the Fairy weakness, removes the Fighting weakness, and adds a Ground weakness in exchange. Adding Steel reduces the Fairy weakness, removes the Flying weakness, but creates a double Fighting weakness while adding in Fire and Ground weaknesses. Scrafty and Pangoro are probably best off with Poison, unless they really really want Burn immunity.
Hydreigon: Adding Fire reduces the Fairy weakness, eliminates the Bug weakness, and adds in a Rock weakness. Adding Poison reduces the Fairy weakness, eliminates the Bug and Fighting weaknesses, and adds no weaknesses. Adding Steel reduces the Fairy weakness, eliminates the Bug and Dragon weaknesses, and doubles the Fighting weakness. Overall I'd add Poison myself.
Seriously just add Steel, never look back. As for its buddies, Rayquaza, Salamence, Altaria, and Noivern? Same thing. Add Steel, never look back, don't care that they don't all have Steel moves like Dragonite. (Mostly this means they're bad Dragonites in this meta)
Dragon/Ground: Adding Steel to Ground/Dragon is a little different, adding both a Fighting and a Ground weakness while covering up the Fairy and Dragon weaknesses and reducing the Ice weaknesses. It's nice to not have a double weakness though? Adding Water adds a Grass weakness, places you at x8 damage from Freeze Dry (But hey you're only one stage vulnerable to regular Ice), and doesn't really do much else unless you really want a double Fire resistance or something. Adding Fire adds a Ground and Water weakness, eliminates the Fairy weakness in addition to reducing the Ice weakness, and provides immunity to being Burned, which is important to all three Ground/Dragon Pokemon. And of course Adding Ice is pretty sub-optimal, though it actually only needs a Steel and Fighting weakness here. Mostly Zygarde (And Flygon, and Garchomp) must look enviously upon Dragonite (And its buddies), wishing they could be as awesome. Flygon can actually learn Fire Punch, so it should give serious thought to adding Fire. Of the three, Flygon also has the advantage that it gets to ignore the Ground vulnerability, leaving the enemy to guess whether your Flygon is vulnerable to Fighting or Water if they don't have Ice to pick on it. (Though they really should have Ice, just because Dragonite is kind of ridiculous)
Grass/Flying: Adding Steel to Grass/Flying results in a double weakness to Fire (same for adding Ice), adding in Fire results in a double Rock weakness, adding in Water results in a double Electric weakness (And a triple Freeze Dry weakness)... Tropius, Jumpluff, and Shaymin-Sky cannot escape having a double weakness, and none of them have Abilities to soften the blow. Nonetheless, adding Fire or Water may be worth it just because Ice has priority in the form of Ice Shard, while Electric and Fire are currently incapable of priority, which can make a big difference for Shaymin-Sky and Jumpluff in particular, who normally rely on their speed to help protect them.
Torterra: Adding Steel is again a double weakness to Fire (And again Ice is too), but adding in Water actually adds no weaknesses while not only reducing the Ice weakness but eliminating the Fire weakness. (That triple weakness to Freeze Dry though) Adding Fire on the other hand reduces the Ice weakness, eliminates the Fire and Bug weaknesses, renders Torterra immune to Burns, and the only weakness it adds is to Water. (It also avoids Freeze Dry dooming you) Either one could be good for Torterra.
Ground/Flying: Adding Steel reduces the Ice weakness at no real cost. (Not so much adding Ice) Unfortunately, this combines poorly with Gliscor's desire to utilize Poison Heal, but Landorus is probably perfectly happy to take Steel. Adding Water reduces the Ice weakness and eliminates the Water weakness, but adds in a Grass weakness and brings into play the Freeze Dry=doom problem. Lastly, adding Fire reduces the Ice weakness, adds a Rock weakness (Including Stealth Rock!), and turns the Water weakness into a double weakness. But hooray for Burn immunity? (Gliscor would rather just take Poison Heal and add Water) Basically, Landorus should probably just take Steel, while Gliscor should probably just take Water and be ready to drop everything and flee from anything that can get Freeze Dry.
Fire/Flying: Adding Steel reduces the Rock weakness, with the only cost being if you want to Roost and something Earthquakes you or similar. Adding Fighting also reduces the Rock weakness, but adds in a Flying and Psychic weakness, though it doesn't add any situational double weaknesses either. Adding Ground adds an Ice weakness, doubles the Water weakness, and OK yeah reduces the Rock weakness. (Which is maybe a worthwhile trade if Stealth Rock is your big concern) Mostly, Charizard, Talonflame, Moltres, and Ho-oh should just add Steel and not look back. (Unless we're talking Mega Charizard X, which is a whole different thing) To be fair, Mega Charizard Y often runs Focus Blast anyway, so having STAB to really wreck Heatran would be nice... on the other hand, Heatran is generally going to be Flying anyway, so it won't actually be vulnerable!
Volcarona: Adding Steel adds a Ground and Fire weakness, but reduces the Rock weakness, and removes the Flying weakness. Decent trade. Adding Fighting reduces the Rock weakness, but gives it a double Flying weakness and adds in a Psychic weakness hooray don't do it. Adding Ground reduces the Rock weakness, doubles the Water weakness, and is just kinda not good. Basically, unless you want to just accept the double Rock weakness (And it's a huge part of what holds Volcarona back), tack on Steel, however tempting it might be to do something like add Grass for Giga Drain STAB. (Which results in another double Flying weakness, by the way)
Camerupt: Adding Water adds a Grass weakness, reduces the Water weakness, and that's about it. (But you're weak to Freeze Dry) Adding Grass reduces the Water weakness, removes the Ground weakness, and adds in a Flying weakness. Adding Dragon reduces the Water weakness, and adds a Dragon and Ice weakness, hooray. Grass is actually probably it's best option, being a net positive in effectives and throwing in immunity to Spore et al for free. This of course all applies to Simple Eviolite Numel too.
Magcargo: You can bypass its double weakness to Ground with Flying, actually, so one could argue it belongs in the first category, but its double weakness to Water cannot be erased with some other type. Now, there's good news: adding Grass reduces both its double weaknesses to single weaknesses, with no added weaknesses! (And throwing in immunity to Spore et al to boot) No other type resists both Water and Ground at the same time, but that's not so bad when adding Grass is so perfect. Nonetheless, for your edification: you can add Flying to be immune to Ground but go up to doubly weak to Rock and just no ugh, you can add Bug to reduce the Ground weakness but again become doubly weak to Rock and ugh, you can add Water to reduce its Water weakness but gain both Electric and Grass weaknesses, or you can add Dragon to pick up a Dragon weakness while reducing the Water weakness and it's actually kind of nice overall. But mostly: add Grass, put it on a Sun team (Now your Water weakness is truly neutered!) give it Solar Beam, be amazed at Magcargo actually not completely sucking!
Or probably still don't use it because it's still kind of mediocre and its Abilities in particular are sad.
Water/Rock: Adding Grass reduces the Grass weakness, removes the Ground and Electric weaknesses, in exchange adding a Bug weakness, and making Freeze Dry murder you. Adding Bug reduces the Grass weakness, removes the Ground and Fighting weaknesses, in exchange adds a Rock weakness, and is OK generally. Adding Steel results in double weakness to both Ground and Fighting why would you do this to yourself? Adding Poison results in another double Ground weakness while adding a Psychic weakness too, though to be fair it actually both reduces the Grass weakness and also eliminates the Fighting weakness, but even so. Adding Flying results in a double Electric weakness, hooray, and a double weakness to Freeze Dry, double hooray. Adding Dragon reduces the Grass weakness and eliminates the Electric weakness, throws in Fairy and Dragon weaknesses, and again makes Freeze Dry murder you. Anything else does nothing about the double Grass weakness. Overall, Barbaracle, Relicanth, Carracosta, Omastar, Kabutops, and Corsola are probably best off with either Bug or Grass.
Ground/Water: Adding Grass reduces the Grass weakness, adds in an Ice weakness (And thus raises Freeze Dry from x4 to x8 damage) plus Flying and Bug weaknesses, but hey immunity to Spore and all? Adding Bug reduces the Grass weakness, adds in a Flying weakness, and that's pretty OK. Adding Dragon reduces the Grass weakness while adding in Fairy, Ice, and Dragon weaknesses, which includes Freeze Dry murdering you, don't do it. Adding Poison reduces the Grass weakness while adding in both Psychic and Ground weaknesses, which is maybe worth it? Adding Steel reduces the Grass weakness while adding in Ground and Fighting weaknesses. Keep in mind it also reduces the Freeze Dry weakness to one tier. Adding Flying reduces the Grass weakness but adds an Ice weakness including of course increasing Freeze Dry's instagib potential. Overall, Swampert, Gastrodon and Seismitoad are probably best off adding Bug or Steel, or maybe Poison if you have too many Fighting weaknesses as is on the team. Gastrodon finds Grass even less appealing than usual, since the resistance to Water is provides is largely irrelevant when it's probably running Storm Drain anyway.
Grass/Psychic: Adding Ghost reduces the Bug weakness but redoubles the Dark and Ghost weaknesses. No. Adding Flying reduces the Bug weakness, but adds in a Rock weakness and doubles the Ice weakness. Adding in Poison reduces the Bug weakness, removes the Poison weakness, and adds no new weaknesses. Very nice, especially since it throws in Toxic immunity. Adding in Steel reduces the Bug weakness and eliminates the Ice weakness, but doubles the Fire weakness. Eeeeeh. You can't add Fairy, and it would be kind of terrible to add anyway. Adding Fighting reduces the Bug weakness and eliminates the Dark weakness, but doubles the Flying weakness and adds a Fairy weakness. Lastly, adding Fire reduces the Bug weakness, removes the Fire and Ice weakness, but adds in a Rock weakness. Overall Poison and Fire are, by far, the best things to add to Celebi or Exeggutor.
Malamar: Adding Ghost reduces the Bug weakness but adds in Dark and Ghost weaknesses. Adding Poison reduces the Bug weakness, removes the Fairy weakness, and only adds a Ground weakness in exchange. Quite nice. Adding in Fighting reduces the Bug weakness, doubles the Fairy weakness, and adds in a Flying weakness too. No. Adding in Fire reduces the Bug weakness, removes the Fairy weakness, and adds in Ground, Rock, and Water weaknesses. So a bad Poison. Adding in Flying reduces the Bug weakness, but adds in Rock, Electric, and Ice weaknesses. Awful. Still can't add Fairy, though it would actually be kind of OK if you could. Lastly, adding in Steel reduces the Bug weakness and removes the Fairy weakness, but adds in Fire, Ground, and Fighting weaknesses ie again a bad Poison, particularly since they both provide immunity to Toxic et al but Poison lets you absorb Toxic Spikes. If you're going to use Malamar, there's only really two routes: add Poison, or add Fighting and accept the double Fairy weakness in exchange for Contrary Superpower STAB.
Cacturne: Adding Ghost reduces the Bug weakness and eliminates the Fighting and Poison weaknesses with no added weaknesses. Nice. Adding Poison reduces the Bug weakness, removes the Fighting and Poison weakness, and also adds no new weaknesses. Adding Fighting reduces the Bug weakness, doubles the Fairy and Flying weaknesses... no. Adding Fire reduces the Bug weakness, eliminates the Fire and Ice weaknesses, and only adds a Rock weakness in exchange. Decent. Adding in Steel reduces the Bug weakness, removes the Ice and Poison weaknesses, but doubles the Fire and Fighting weaknesses. Ugh. Adding in Flying reduces the Bug weakness, eliminates the Fighting weakness, adds a Rock weakness and doubles the Ice weakness. Eugh. Overall Cacturne is probably best off with your choice of Ghost or Poison (Ghost is straight up immune to Fighting, but Poison cannot be Poisoned), or maybe Fire if it better suits your team or if Cacturne becomes popular enough people start assuming that it's Ghost or Poison and just hit it with Fire or Ice.
Aurorus: Like Magcargo, Aurorus can eliminate one of its double weaknesses, but not the other. Adding Ghost will of course completely eliminate its double Fighting weakness, but it has no effect on its double Steel weakness and adds Dark and Ghost weaknesses. There is in fact no single type that resists both Fighting and Steel: if you add Steel, Water, Electric, or Fire to reduce the double Steel weakness, the double Fighting weakness is unaffected, while adding Ghost, Flying, Bug, Poison, Psychic (Or Fairy, but you can't do that anyway) has no effect on the double Steel weakness. (Except Fairy would actually make it even worse) Bonus points: adding Water doubles the Grass weakness, adding Electric (Which it actually has STABs for) Poison or Fire doubles the Ground weakness as does Steel while also bring the Fighting weakness up to triple weakness, while adding Flying or Bug doubles the Rock weakness. Only Psychic and Ghost don't replace the double weakness they're clearing with at least one other, and of the two Ghost is basically flatly superior. In short, Aurorus is still screwed.
Bug/Flying: Alas, that there are only three types that resist Rock. Adding Fighting results in a double Flying weakness, adding Steel results in a double Fire weakness, and adding Ground results in a double Ice weakness. Of these, Steel (Adds no other weaknesses) and Ground (Also adds Water as a weakness, fewer resistances than Steel) are probably your best bet for Butterfree, Beautifly, Vespiquen, Mothim, Masquerain, Ninjask, Yanmega, and Vivillon. Or you could ignore the problem entirely, but a double weakness to Rock is problematic when that means Stealth Rock can 2HKO you on its own. In short: Bug/Flying is still an awful typing and you probably shouldn't use any of its representatives even in this meta without a really good reason. Mostly: add Steel, and plan around it. (Note that Vivillon's access to Powder can punish attempts to incinerate it, or psych people out into not using it at all)
Ice/Flying: Like the Bugs, but the news is a little better actually. Adding Steel will still result in a double Fire weakness, and also add in a Fighting weakness while its at it, but Fighting results in no double weaknesses (It still adds all of Psychic, Flying, and Fairy to your list), and Ground too results in no double weaknesses. (Adds Water and Ice weaknesses) Fighting and Ground being added are both viable for Articuno and Delibird. Now, whether either of them is viable is a whole other matter...
Bug/Steel: The only thing to deal with is a double Fire weakness, and I'm not going to repeatedly state its being reduced. Adding Water adds in an Electric weakness. Adding in Dragon adds no weaknesses at all. Adding in Fire adds a Ground, Rock, and Water weakness, don't do it. Adding in Rock adds a Fighting and Water weakness. In short, Forretress, Scizor, Escavalier, Wormadam-Trash, and Genesect should all add Dragon and never look back unless your specific goal is to acquire STAB on a move. (As if Forretress cares)
Ferrothorn: Adding Rock reduces the Fire weakness but makes the Fighting weakness worse. Adding Dragon reduces the Fire weakness but brings in an Ice weakness. Adding Water reduces the Fire weakness but adds a Freeze Dry weakness. Adding Fire reduces the Fire weakness but adds in Ground and Rock weaknesses. Overall Ferrothorn is probably best off adding Water in practice.
Abomasnow: Adding Rock reduces the Fire weakness, removes the Poison and Flying weaknesses, but doubles both the Steel and Fighting weaknesses. Eugh. Adding Dragon reduces the Fire weakness, adds in Ice, Dragon, and Fairy weaknesses, and that's it. Adding Water reduces the Fire weakness, removes the Steel weakness, and adds no new weaknesses. (Except Freeze Dry) Adding Fire reduces the Fire weakness, removes the Bug and Steel weaknesses, but doubles the Rock weakness. (Oh god Stealth Rock) Add Water, don't look back, basically? Unless you're personal goal is to remove the Stealth Rock weakness I guess.
Bug/Grass: With a double weakness to both Flying and Fire, this is torture. Fortunately, adding Rock reduces both of those and also removes the Poison weakness, with its only cost being adding in a Steel weakness, because Bug/Grass resists or double resists all of Rock's other weaknesses. Very nice. Alternatively, you can add Dragon (double Ice weakness, Fairy Weakness, Dragon weakness, only reduces Fire damage taken), Fire (Double Rock weakness, though it also removes the Bug weakness), or Water (No added weaknesses and removes the Ice weakness, barring Freeze Dry) to reduce the Fire weakness while doing nothing about the double Flying weakness, or you can add Electric (No added weakness) or Steel (Makes the Fire weakness even worse, though to be fair it also clears the Poison, Bug, Ice, and Rock weaknesses, leaving you with just a Flying weakness and a triple Fire weakness) to reduce the Flying weakness without addressing the double Fire weakness. The general best choice is Rock, but Steel might be the true best choice for Parasect, Wormadam-Grass and Leavanny, since it narrows them down to just two weaknesses. Just beware... you know... anything with so much as Ember (Especially Dry Skin Parasect) and you'd be surprisingly decent.
Grass/Fighting: Adding Rock reduces the Flying weakness, removes the Poison weakness, and adds in Fighting and Steel weaknesses. Adding Electric reduces the Flying weakness with no added weaknesses. Adding Steel reduces the Flying weakness, removes the Poison, Psychic, Fairy, and Ice weaknesses, but makes the Fire weakness a double weakness and adds a Fighting weakness. The general best choice for Breloom, Chesnaught and Virizion is probably to add Electric, but Steel can be a worthwhile trade-off and for Virizion it may be worth going for Rock to gain STAB on some of its only off-type moves. Keep in mind Chesnaught's Bulletproof can also let it shrug off some weaknesses more readily than others, though nothing stands out to me as strongly relevant unfortunately.
Ground/Rock: Double weakness to both Water and Grass hooray. Good news: adding Grass reduces both of these weakness at the same time! Bad news: it leads to a double Ice weakness and adds a Bug weakness. Adding Dragon also reduces both weaknesses and also results in a double Ice weakness while adding in Fairy and Dragon weaknesses. There's just no escaping a double weakness for Golem and Rhyperior. Even so... adding Water reduces the Water weakness, removes the Ice weakness, and leads to a triple Grass weakness and a double Freeze Dry weakness. Adding Flying reduces the Grass weakness, removes the Fighting and Ground weaknesses, doubles the Ice weakness yipee. Adding Bug reduces the Grass weakness and removes the Fighting weakness with no added disadvantage. Adding Fire reduces the Grass weakness, doubles the Ground weakness, leads to a triple Water weakness, and is just awful. Adding Poison reduces the Grass weakness and removes the Fighting weakness, but the Ground weakness is doubled and a Psychic weakness is added. Lastly, adding Steel reduces the Grass weakness, removes the Ice weakness, and leads to a double weakness to both Ground and Fighting. Ground/Rock has the dubious honor of being the first type combination I've found can give itself three double weaknesses by adding a type. Ugh.
Fighting/Dark: Adding Fire reduces the Fairy weakness, but adds in Ground and Water weaknesses. Adding Poison reduces the Fairy weakness, removes the Fighting weakness, and adds a Ground weakness in exchange. Adding Steel reduces the Fairy weakness, removes the Flying weakness, but creates a double Fighting weakness while adding in Fire and Ground weaknesses. Scrafty and Pangoro are probably best off with Poison, unless they really really want Burn immunity.
Hydreigon: Adding Fire reduces the Fairy weakness, eliminates the Bug weakness, and adds in a Rock weakness. Adding Poison reduces the Fairy weakness, eliminates the Bug and Fighting weaknesses, and adds no weaknesses. Adding Steel reduces the Fairy weakness, eliminates the Bug and Dragon weaknesses, and doubles the Fighting weakness. Overall I'd add Poison myself.
It wouldn't surprise me if I missed some double weakness combo or another, but I'm pretty sure this is everything.
Freeze Dry might actually become important in this meta, depending on what Pokemon and specifically what types they prefer to add end up rising to prominence. So Aurorus, Articuno, Cryogonal, Delibird, Lapras, Mamoswine and Vanilluxe might rise in prominence for that alone.
