
Tera Types:
Water*, Fighting^, Psychic^
Barraskewda is a Pokemon that can be fierce under rain, but I think Terastallizing could be used better on Rain teams. Thing is that Barraskewda already hits hard with its Water STAB and has good coverage, so it wouldn't need it as much, and Tera Grass is surprisingly a great Tera type to have, along with Water.

Tera Types:
Dark*", Steel"*, Flying"^, Ghost"
Bisharp is more of an ideal user of Terastallizing. It would utilize the mechanic both offensively and defensively. The coloration here looks weird, but because of the use cases for each type. Like for example, you more often will use Dark on Bisharp on more offensive teams because Dark is a pretty great offensive typing and is decently defensive as well, but defensively is more niche. Dark being one of the better Tera types for Bisharp since Dark STAB is only resisted by Fairy. Steel is an alternative, but more if you care about Bisharp's defensive utility while making Iron Heads incredibly strong (at the expense of Dark STAB). Flying would mostly be used defensively since Flying compliments Bisharp well, but offensively would require you to use Tera Blast and any Pokemon with Tera Blast has to be the dedicated user of Tera types on teams, but would be useful against Fighting. Lastly, Ghost would be better than Flying defensively, but would purely be defensive since Dark pretty much does the same job anyways.

Tera Types:
Fire*, Ghost*", Ground^", Fighting^, Ice^, Dark"
Blacephalon is very much a niche user of Terastallizing. It's because it's not lacking in power by any means and Terastallizing it is very risky due to its low survivability. The best you can do is super power one of your STABs, give yourself coverage for Tyranitar, Heatran, and Dragonite, or weaken a Shadow Sneak/Sucker Punch so you aren't immediately revenge killed. But overall, you want to use something else.

Tera Type:
Ghost", Fairy", Water", Steel", Poison", Flying", Ground", Grass", Really any type besides like Ice, Bug, and Rock
Blissey is one of the Pokemon that owns Terastallizng. It's a very customizable Pokemon suited for many teams that need to wall out any Pokemon it so desires on the special side. It normally does this, but now it can be more effective at walling special attackers and can use Tera Types to have better match-ups against some physical attackers. Most notable being the Fighting type moves like Close Combat most Pokemon have. Ghost and Fairy alleviate its original weakness of Fighting, although while also making it weak to other types. Ghost and Fairy definitely compliment Normal the most, but most other types work on Blissey depending on your team. Sadly, there isn't much use of Terastallizing Blissey for offensive reasons.

Tera Type:
Steel", Electric"^, Fighting*
Buzzwole is another good user of Terastallizing. It has a unique typing making it useful as a tank thanks to its resistances. However, Buzzwole does suffer from a pretty exploitable typing with it being x4 weak to Flying, and also Fairy/Psychic/Fire. Thankfully, it has Steel and Electric to compliment those Weaknesses, especially Steel. Electric is notable because of Thunder Punch STAB against Flying types. Additionally, Buzzwole can use Fighting Super STAB effectively on Choice Band sets.

Tera Type:
Fire"^, Steel", Fairy*, Electric"^, Ground"^
Clefable is an excellent user of Tera Types due Magic Guard. With Magic Guard, the user is capable of fully using the defensive utility of Fire and Flying without being Stealth Rock weak. On Clefable, Fire is probably the best Tera Type is can have because of this. It compliments Clefable well as it resists Steel and is neutral to Poison, but also keeps the resistance to Bug, resists Fairy/Ice/Grass/Fire, and gives Clefable STAB on Flamethrower/Fire Blast to threaten most Steel types. Steel is also pretty useful as it resists Steel and is immune to Poison, however it's now weak to Fire/Fighting/Ground. Still, can't go wrong with the best Defensive typing in the game. Fairy can be useful to give it effective Fairy Aura, and while it's good defensively, Clefable is Mono Fairy already so it has no defensive benefits. Electric is similar to Fire but more niche. And Ground can be useful defensively, but if you want to use it offensively Clefable either needs to learn Earthpower in SV or use Tera Blast. If Clefable learns Earthpower, Ground becomes between Fire and Steel.

Tera Type:
Dragon", Ground"
Corviknight isn't too interesting of a Tera user, but if some OMs have taught me is that Dragon and Ground compliment Flying/Steel pretty well. Not only that, but in case you need to escape Magnezone, you have this option as well.

Tera Type:
A Lot^"*
Dragapult is a Pokemon that already is borderline broken in OU already. Almost all metas that even slightly buff it ban Dragapult almost immediately because it's just on the edge of perfection by OU standards. Pokemon that are customizable are the best users of Terastallizing and Dragapult is a prime example. It might be a cop out to just say "A Lot" but going over how Dragapult can utilize different Mono types would be too wild. It has a wide arrange of coverage options and making its STABs strong is
TERAfying terrifying. In SV, if it is in the Dex, I expect Dragapult to be PU (Possibly Ubers).

Tera Type:
Water"^, Steel"^, Ground"^, Fire"^, Poison", Fairy"
Yet again, we have a Pokemon that is a great user of Terastallizing due to its diversity, but not to the extent of Dragapult. It's also great because of Multiscale and its bulk. It makes Dragonite incrediblly hard to kill before it gets multiple boosts up, and Dragonite doesn't really mind forgoing STAB because it can muscle through most match-ups. Water is one of the best Tera types for Dragonite
Foreshadowing because it resists it's archenemy Ice types. Additionally, Water gives Dragonite STAB on Aqua Tail/Waterfall. Steel has a niche as well for giving Dragonite a resistance to Ice AND Fairy, and also gives Iron Head STAB, but I say that Water has better STAB generally. If you are looking for purely defensive Dragonite, use Steel, Poison, or Fairy. Ground is also useful for Earthquake and is good defensively, but is weak to Ice. Fire is similar to Water, but you are still Rock weak and Fire Punch isn't as good as Aqua Tail or Aqua Jet.

Tera Type:
Fire", Water", Ghost", Fairy", Grass*"
Ferrothorn is now its own FireWaterGrass core on its own as it can use the starter typings pretty well, as well as Ghost or Fairy. Ferrothorn's usage with Terastalling is a bit moderate and more situational, like Fire being more for something to let Ferrothorn not be useless against Sun teams while also letting it escape Magnet Pull. Water does the former while being a good type in general. Ghost and Fairy let Ferrothorn deal with Fighting types when it has nothing else to really deal. Grass actually has a niche as Ferrothorn actually has a pretty scary Power Whip and would still like to resist Water and now resist Ground and be neutral to Fighting. It's still niche as Ferrothorn still likes using its original typing.

Tera Type:
Water"^, Steel"^, Ground*", Fire^"
A pretty infamous Pokemon that has been a staple of OU since Gen 5 after being unbanned. Of course one of the most influential Pokemon of all type will use this mechanic well. Resisting Ice of course is a neat bonus, especially with its coverage moves like Aqua Tail and Iron Head. Additionally, Super STAB Earthquake makes it incredibly strong. Fire here is more so for Garchomp to beat Skarmory/Corviknight while also resisting Ice, but you become Stealth Rock weak and makes your match up against Landorus-T worse.

Tera Type:
Grass", Flying", Fairy", Ghost", Ground"^, Fire*
Another OU Staple that changed competitive forever. Heatran is great because of its original typing, but that doesn't mean it won't use Terastallizing well as it can patch up its exploitable weaknesses. Grass is by far the best Tera Type for Heatran. Not only does Grass compliment Fire/Steel well, but Heatran's ability in Flash Fire. This means Heatran will beat out other Heatrans in Heatran wars. While the same can be done with Flying while making you resist Fighting, Grass has the benefits of resisting Water and Electric. Water being an important resist due to many Pokemon wanting to become Water and so it's not dead weight against Rain teams. Fairy is a pretty neutral type that resists Fighting and appreciates the original Heatran typing. Same with Ghost. Ground is also useful in winning Heatran Wars but also lets it resist Rocks and because Earthpower is pretty good, though you are weak to Ice/Water/Grass. Last, is if you are running a Sun Team, Fire Heatran spamming Terastallized Eruptions/Fire Blasts under the Sun would be fun, but could be more useful on something else.

Tera Type:
Rock^, Electric^, Flying^", Fighting^, Dark^, Grass*, Steel*
Kartana is a weird Pokemon with Tera Typing. Since Kartana is one of the few Pokemon you would want to have Tera Blast on, Rock, Electric, and Flying (assuming TB is stronger than Aerial Ace) would naturally make it more of a dedicated Terastallize user. This designation is only given to Pokemon that do so and Kartana does it three times with its preferred typings. Rock is good against Flying and Fire types. Electric is also good against Flying, but also Pokemon like Skarmory and Corviknight. Flying has the benefit of resisting Mach Punch while assisting it against bulky Grass and Bug types. You can also give yourself STAB and Super STAB. Despite honestly not being that interesting and being more dedicated, Kartana will like be PU (possibly Ubers) because it's also a nearly broken Pokemon.

Tera Type:
Water", Flying*, Ground*
Smogon Lion being blessed with ways to rid itself of its Ice weakness and it's Water weakness too. Water is definitely the way too go with defensive Landorus-T when the situation calls for it. If you want to, you can also run Flying with Tera Blast to give yourself great Flying/Ground coverage, or give your Earthquakes Super STAB. Other than that, there isn't much else in how to use Landorus-T.

Tera Type:
Flying", Fighting^
Honestly, Magnezone seems broken on paper, but is probably one of the most nerfed Pokemon thanks to this mechanic. It's sole job is to eliminate Steel types, which is made much harder when many Steel types can literally just flee and Tera Steel types like Garchomp would just rip Magnezone in half with Earthquake. Additionally, Terastallizing Magnezone is just terrible in most scenarios since Magnezone is meant to be a one trick pony, not last the entire game. If you do use it, use Flying or Fight with a Body Press set in hopes to do some damage. Just don't use it.

Tera Type:
Steel*
Metal Melmetal doubling down on Double Iron Bash is probably the best you can really do with Melmetal. It's already good as is and Terastallizing doesn't do much and in fact hinders its ability to spam DIB and being a tank in general. It's already practically invincible physically and would only really benefit from using Water, but you lose DIB STAB doing that.

Tera Type:
Literally 600 BST Arceus"^*
You know, this might be another copout, but you can literally just make Mew into anything. It's by far the most diverse Pokemon to use Tera Type and can be useful on any team similar to how Arceus is used.

Tera Type:
I literally had to make a new tier of usage since Terastallizing Alolan Ninetales is one of the dumbest things you can actually do. But in if there was a scenario for Terastallizing Alolan Ninetales, it would be Dark or Steel
Literally never Terastallize Ninetales. You are throwing the game if you do. Even the scenarios like avoiding Prankster or maximizing bulk in the late game, just DON'T.

Tera Type:
Grass",
Ground", Water*
Ok, now back to something potentially useful. Pelipper is the most important unit to Rain teams, and making it survive as long as possible to set Rain as much as it can is key to its success, but that's about it. Grass and Ground help with the Electric weakness so it can survive longer. Water can make it's Hydro Pump stronger and Stealth Rock neutral, but you're better off using other Pokemon for that.

Tera Type:
Grass^",
Ice^, Electric*, Dark"^, Ghost"
Regieleki is pretty unique as a Tera abuser, but honestly it's pretty weak. Grass/Ice lets it beat Ground types like Hidden Power would have before, Grass being preferred since it's better defensively than Ice, though Ice handles Landorus-T better. Electric is literally just a meme but can work if you can remove Ground types. Ghost/Dark has a niche as anti-priority on Screens, which you really shouldn't do, and you at least get STAB on Assurance. Definitely going to be a noob trap as people still don't understand Rampardos Theorem or the value of defenses on even offensive Pokemon (side note, you should go play Advance Wars By Web. Defense is more important there due to a number of factors, but you'll get a new appreciation of defense you otherwise wouldn't. Also it's fun).

Tera Type:
Grass*, Fighting^, Dark^
Monkey swing! Rillaboom is a star pivot thanks to Grassy Terrain, good bulk, U-turn, and being pretty threatening. It honestly be decent Terastallizing, but doesn't need. It is pretty hard thinking of a good typing since Rillaboom should be grounded and the appeal is stronger than Talonflame priority+even Stronger regular STAB. Other than making Grassy Glide even stronger, it can have a stronger Fighting STAB or Knock Off (but as great cost to monkey)

Tera Type:
Grass", Steel", Dark"
Slowbro is a nice pivot, but mostly because of its typing. The best ones are the Tera Typings that resist its weaknesses, which is actually pretty hard to do.

Tera Type:
Dark"^, Psychic*
Slowking-Galar really doesn't want to Terastallize since it really doesn't have that great of options. Dark is what I think would be the best type for it since it resists Ghost and Dark, and also gives Foul Play STAB if you want to. Psychic is also an option as Slowking is actually a pretty good offensive Pokemon with Nasty Plot.

Tera Type:
Grass"^, Steel"
Tapu Fini is also not a great Terastallizer either. It's most useful type is Grass to resist Grass and Electric, and additionally can use Steel for Grass and Poison, especially Slowking-G. Thing is Tapu Fini isn't a Pokemon that would benefit from becoming Mono or having a typing that is immune to Toxic, which it already has with Misty Terrain. It does gain STAB on Grass Knot, which it doesn't really need, but it is there.

Tera Type:
Grass"^, Electric*, Flying^", Ice^
Tapu Koko is a much better example of a Tapu using Tera Types to great effect, like most Electric types. Tapu Koko naturally enjoys a ways to beat Ground types, and Grass is the best option due to Grass Knot, and the only other Ground counter type it would use is Flying. Super STAB with Electric Terrain is also great on Tapu Koko due to it being an Electric type with actual coverage. Flying is niche for when you want to spam Brave Bird to beat Grass types with natual STAB, but does mean you lose out on your own Terrain. Additionally, you can use Ice to beat Ground/Grass/Dragon, though it means you lose a ton of defensive utility on a pivot and Tapu Koko has Brave Bird and Dazzling Gleam anyways.

Tera Type:
Psychic*, Fire"^
Tapu Lele is a pretty decent user of Tera Types. It doesn't need much, just Psychic spam which it already does well. However, there is a case for using Fire as it both is a way to beat Steel types both defensively and making it more reliable than Focus Miss.

Tera Type:
Steel", Ground", Flying*
Tornadus-T is a pretty good user of Tera Types. Being a Mono type Pokemon that is also fast and has Regenerator, it doesn't lose much while also gaining so much. It can use it on defensive pivot sets or offensive Nasty Plot sets as well. Steel resists Ice and Rock, while Ground resists Rock and is immune to Electric. Offensively, making Tornadus Flying doesn't do anything defensively, but since it is Mono Flying, there isn't a downside to not using Flying on NP sets.

Tera Type:
Grass", Dark"^
Toxapex is a decent user of Tera Types, but does fall a bit flat since you do lose its great typing. But still, Terastallizing into a Grass or Dark type can be pretty useful in some scenarios.

Tera Type:
Rock*, Ghost", Fairy"
Tyranitar is probably one of the worse Terastallizers despite the benefit of becoming a type that beats Fighting. This is because of course Sand Stream. This is probably the only time a non-Tera Blast user would be "dedicated" Terastallize user solely because using it for Ghost or Fairy means you would need Unnerve, which is just bad. Additionally, despite looking bad on paper, Tyranitar's typing is actually somewhat good defensively. Mostly because it resists Ghost, Dark, Psychic, Flying, and Fire, all of which are pretty good resists considering its x1.5 SpD in Sand. If Tyranitar didn't have Sand Stream, it would be probably UU, but would have a completely different story.

Tera Type:
Water*"
Urshifu-R is definitely the best user of Tera Types on Rain teams. Pokemon like Pelipper and Barraskewda are secondary to how threatening Urshifu-R can be with Super STAB on Surging Strikes. It's both much more powerful and bulkier. The power under Rain is definitely worth the lose of Fighting STAB. If you don't plan on using it on Rain, you might want to hold off on using it if your team otherwise would need to use it.

Tera Type:
Fire*, Electric^", Grass"^, Fighting"^
Like Mew, Victini is 100 across the board, but with a movepool that is somewhat lacking in utility. Yeah, it has great coverage and also STAB v-Create, but it is pretty linear in what it does, which is fine for Victini. Since Victini rarely uses its Psychic STAB, you can use Fire to make V-Create go Brrr. This is nice on Victini as not only is V-Create even stronger, but it also has Bolt Strike. Speaking of which, Victini can give Bolt Strike STAB to make that stronger while still have a powerful V-Create and not being Stealth Rock weak either. Grass, again, is great for Ground and Water types and lets it use its Grass coverage too. Additionally it can tap into its Fighting coverage while resisting Rock/Stealth Rock.
But I would like to take this time to show off, while powerful, Super STAB isn't broken.
Here is Sun Boosted, Choice Band, Super STAB V-Create on a Water Avalugg
more Foreshadowing
252 Atk Choice Band Adaptability Victini V-create vs. 252 HP / 252+ Def Avalugg in Sun: 170-201 (43.1 - 51%) -- 3.9% chance to 2HKO
Even after giving V-Create an effective 810 BP after tons of set up, it still is less than likely to 2HKO Water Avalugg. This is a ridiculous scenario as Victini would need set up and a less than desirable item to barely 2HKO Avalugg (after becoming Water). This is why you can't really apply Super STAB toeverything and call it a day. Victini is unique among Pokemon that would do this because it has great coverage to surpass Water Avalugg, Toxapex, Heatran (if it isn't Grass), Tyranitar, Volcanion, and Water Landorus-T. Regieleki is noob bait because its coverage is awful.
I wanted to mention it here since Victini is a good example of why this is the case with an extreme case.

Tera Type:
Ground"^
Volcanion is unique as a Water/Fire type, but is further unique as it's the only Water and Fire type that doesn't really care to have Grass as a Tera Type, which is pretty ironic. This is because Volcanion's ability Water Absorb does the main job Grass has on Fire types and Ground is flat out immune to Electric. Not only that, but Volcanion now resists Rock and gets STAB on Earthpower.

Tera Type:
Psychic^, Water^", Ground^"
Volcarona is back at it again, where while still good, people are mostly shitting themselves over nothing. Volcarona's history is funny like that. Gen 7, it'll be OP because of Z-moves. Gen 8, it'll be OP because of Heavy Duty Boots. And now Gen 9, and people are scared of what is essentially glorified Hidden Power. Though, Water and Ground would benefit Volcarona defensively, you do need to use it as a dedicated Tera user to possibly beat on of its checks and at the expense of your own Fire/Bug STAB. A thing of note is that this is a match up Flying Heatran prefers over Grass Heatran, but because of its original Bug STAB. Also it can use Psychic on Toxapex.

Tera Type:
Ice*, Dark*"
Weavile is a simple Pokemon with typings. Ice/Dark is inherently good offensively, which is why it's another Pokemon you shouldn't Terastallize since Weavile prefers both STABs. Super STAB Triple Axel is scary until you think about the that Victini calc before. Dark is similar with Knock Off, Beat Up, and unironically making Weavile a better type defensively.

Tera Type:
Steel", Water^"
Zapdos is another Pokemon that would prefer not to Tera Type. With Roost, it can already take neutral damage to Ice and Rock, so covering for those weaknesses is not too great. But if you have to, pick Steel since it means faster Pokemon don't get that jump on you and you're immune to Poison. Water can also be used as it resists Ice and gets STAB on Rain Boosted Weather Balls, but like with Barraskewda other Pokemon would use that better.

Tera Type:
Fighting*, Flying*
Sadly another sticker when it comes to Terastallizing, and for the same reason as Weavile. It doesn't really have use for most coverage moves and it likes its STABs too much. If Zapdos-G got Stone Edge, it would be a different story where it could possibly become Rock and not have to use Tera Blast, but I still wouldn't recommend it.

Tera Type:
Grass"^, Electric*, Fighting^, Fairy^", Flying"^, Dark^, Fire^, Steel"^, Ice^
And after a long streak of bad Tera users, we come to what is Terastallizing Royalty. There are so many reasons to Terastallize Zeraora and use an assortment of Tera Types on it. It has everything a the best Terastallize users could want. Great coverage to convert to STAB, get coverage for those new STABs, great pivoting abilities, doesn't lose too much by removing its old typing, and has a built in immunity with Volt Absorb. Not only does it have a actually good defensive utility (with most typings), but imagine trying to fight this thing when it can be any type. Normally, it just runs Plasma Fist+coverage to compliment Plasma Fist. Now, Zeraora can run Fighting Close Combat and have coverage to compliment Fighting or become Grass with Grass Knot and have coverage complimenting Grass. It's nuts since Zeraora is so fast and decently strong with a wide range of coverage. If you want to, you can go with Ice as well with Tera Blast if you really wanted too, and you'd even have some defensive utility against opposing BoltBeam coverage and Tera Blast can be Special or Physical. Not to mention that Zeraora gets Calm Mind and Bulk Up to make it less predictable and more threatening.