Cool thread, not the worst one to be revived. Time to analyze what the worst Mon of each type is from the Competitive and in-game (more Nuzzlocke than 10 old people playing, which is what in-game VRs look like are for here ) perspective.
Grass: Lots of rubbish to choose from. Grass-Bug is a terrible combination, yet the Mons are somehow saved from being the single worst: Parasect has Dry Skin and Spore, Leavanny has passable Speed and SD or Sticky Web. Wormadam-Grass would have been the "winner" of this type, but in SM it got Quiver Dance. Grass-Dark has many weaknesses too, but all of Cacturne, Shiftry and Zarude are really strong offensively (the later 2 fast as well, the former has Sucker Punch and extra immunity), so they can't be the worst either. Grass Dragon it's another terrible defensively combination, but the gimmicks of all of Alolan Egg, Flapple and Appletun give them enough of a niche both in Competitive and in-game. It would have been funny to have a Legendary like Calyrex be the worst Grass Mon, but he also isn't weak enough for that. So, the worst Grass Mon won't be the one with the most weaknesses, but a pure Grass instead, which still gives a lot of options. Cherrim has a cool gimmick for Double Battles, Lurantis has Contrary, Maractus some cool abilities + Spikes, Leafeon has SD and Wish with decent Stats, Tsareena has the ugliest design but surely isn't the worst Mon, Meganium is amazing for Nuzzlockes and isn't useless in Competitive. Which is the worst, then? Among the miriad of physical Grass type Attackers there is one that is both slow and not bulky, while not being terribly strong either. I am looking at you, Carnivine. It's literal Moveset consists in 4 out of SD, Sleep Powder, Synthesis, Power Whip and Knock Off, which doesn't look too bad until you look at the Stars. Why does it have a decent Special Attack and no Movepool there? In the anime it doesn't look so slow, why in the game a f***ing Hippowdon outruns this shit. The ability makes sense, yet is one that doesn't benefit Carnivine except for a Spikes immunity. Poison Heal would improve Carnivine so much and it does make sense on a carnivorous plant (which use several acids to dissolve the insects they catch). From the Competitive point of view, this is for me the worst Grass Mon. Yet, in-game Nuzzlockes I have been able to make this work. What I don't see anyone working is in-game is Sunflora. Another slow and frail Mon with shallow Movepool. Sure, you can use some weird Sun + Trick Room team, especially in a Double Battle. However, in-game you need the following for that:
-Drought Mon. In some games you have an early Torkoal, but Vulpix/Ninetales have the ability as Hidden.
-A good Hidden Power on the useless flower, ideally Fire or Ice.
-Earth Power Move Tutor.
-Trick Room TM, a Mon to learn it and have that Mon with good sinergy with Sunflora and the Drought Mon.
So, it's almost impossible to make Sunflora work in-game and it's clearly the worst Grass Mon there.
Fire: A type in which fast glass cannons and medium speed all-rounders (see Heatran or Arcanine) succeed. So, what we will be looking for is again a slow Mon with low bulk. Flareon and Magcargo certainly are slow and not entirely Bulky, yet they both have Recovery moves and are able to wall and outlast some threats. Both of them even have sweeping potential with Guts Toxic Orb + Flame Charge and Shell Smash respectively. Coallosal doesn't have recovery, but has even more Bulk along with good hazard support. There are also Alolan Marowak and Emboar, but despite their bulk not being the highest, the damage they do is insane. What is left? BW introduced many Fire types, so it is natural that the worst one would be there. Heatmor does hit hard and actually has some coverage (Superpower, Thunder Punch, Giga Drain, some Dark moves), but that is where the positives end. Like Sunflora and Carnivine, it's slow and frail. Ant-Eaters in real life are strong and bulky, sometimes they fight vs jaguars and sometimes they even win against them! Why the Mon is so frail then? I can understand the Speed part, but instead of maximizing both offensive Stars, Heatmor should have higher defenses. Abilities make sense, but better ones are needed to make Heatmor different from other Fire Mons. Originally Heatmor didn't learn Overheat, but nowadays it does, so Contrary would improve this Mon a lot, since it also learns Superpower. In-game, particularly in BW it would certainly help that a Mon with these Stats would appear somewhere earlier than VICTORY ROAD. Seriously, what were the game designers thinking? At least Braviary, Durant, Mandibuzz and Bisharp justify appearing so late by being good Mons, Heatmor should have been present in Lostlorn Forest or somewhere close to there.
Water: For a type with so many Mons, this one is surprisingly easy. I doubt they will ever make a Mon as bad as Luvdisc is. No Stats, no movepool, no unique ability and Lumineon of a things fully outclasses it. In Midele Emerald Hack we made the obvious thing: make Heart Scale double all Stats when held. It became a very strong Mon, but still limited by low HP, shallow Movepool and vulnerability to Knock Off and Trick. Probably in Competitive this would be UU or UUBL Mon. This is what the Pokemon Company should have done if not in 3rd Gen, at least in 6th one, when they started improving some Mons.
Bug: Before BW, this type was designed to have the weakest Mons, after that the philosophy changed and every single Bug Mon created had some useful niche or gimmick. So, the worst Bug has to be somewhere in the first 4 Gens. Illumise could be a good candidate, but it has 2 recovery Moves and can support with Prankster weathers or Thunder Wave. Pure Bug is actually an uncommon typing, way less common in fully evolved Mons than the million of Bug Flyings that... just exist. Yanmega is clearly (with the 6th Gen's Vivillon permission) the best of them, by being powerful with Speed Boost and especially Tinted Lens. A discount Yanmega with lower Stats but Quiver Dance is Mothim. A discount Mothim is Butterfree. A discount Butterfree and the worst of the 2 options Wurmple evolves into is Beautifly. There is no discount Beautifly but there are still Bug Flying Mons. Ninjask influenced some BP Bans in Adv OU and even in later Gens when BP became banned, it still hits hard with Acrobatics and U-Turn, while outspeeding everything. Vespiquen never had such success, but at least it has bulk and some exclusive fun moves. Which brings us to the actual worst Bug, Ledian. Like Luvdisc, it has no Stats at all. The Movepool is very wide, but without Stats or some useful (see:Huge Power) ability, this thing is irredeemable. Sure, it has Dual Screens... like half of the Pokedex. It also has Batton Pass, but apart from not having the bulk to use it, the move has become a TM with a wide distribution. In Midele Emerald this Mon was improved too with Lucky Punch, which doubled the offensive Stats for it. In real games this of course won't be possible (Lucky Punch is a Chansey item), so the only options left are improving the Stats a lot or giving a broken ability like Huge Power or Wonder Guard.
Normal: In first 4 Gens, Ditto is clearly the worst, but then it gained Imposter. Fortunately, there are still a billion of outclassed pure Normals to choose from. Early game rodent would be a good candidate for worst normal and of those, Watchog is by far the worst. However, it doesn't have terrible Stats and the Movepool is quite wide even having SD + coverage for Mons that resist Normal. Castform gets some use in Double Battles with weather teams. Farfetch'd would be another "strong" candidate, but in recent Gens it has been gaining moves, improving Stats and crits now happen more often with Stick. The Mon that never improved since appearing is Delcatty. Terrible Stats across the board dont save the wide Movepool it has. It does have an exclusive ability, but Normalize makes it walled by Ghost. If it gave Scrappy effects too and Delcatty had higher Stats, it could be saved, but right now it's a completely irrelevant Mon that in-game is also very easy to miss, since it appears in very few places.
Poison: Strong and underrated type since the beginning of Pokemon saga. Finding worst Mon here is pretty hard, though it would easily be Beedrill if it didn't gain a Mega. Swalot, despite being outclassed by Muk is still a very bulky Mon with wide movepool. Gardobor has Hazards and wide movepool too. Seviper has SD, Coil and Special wallbreaking potential, with even Trick, Glare and Final Gambit to support the team. Similarly, Arbok has low Stats but useful Movepool and 2 good abilities in Shed Skin . Victreebel is a strong Mon in Sun, while his counterpart Vileplume has good bulk and many support options, even Strength Sap. Maybe we should return to Beedrill... and Mons with the same typing. Scolipede certainly isn't even close to being the worst. Venomoth has Quiver Dance + Tinted Lens, while Dustox has also good bulk and Iron Defense + Roost to boost everything, along with immunity to Freeze. This leaves Ariados, who avoided being the worst Bug, but the bar of not being the worst Poison is much higher. Ariados has Sticky Web, Toxic Spikes, SD and priority moves to combine with the STABs. Yet it's not enough, since those Stats suck, though not as much as Ledian's. It's for sure usable in-game but don't expect it to carry your team, since it's always getting hit before being able to move and it won't take many hits. Higher Stats would be a solution, like for every bad Mon. Another typing such as Bug-Dark, would also help. In-game, Shadow Tag wouldn't improve it much (except when facing it), but in Competitive, Shadow Tag Ariados would be very good... when not banned.
Electric: The curse of being the worst electric certainly belongs to the pikaclones, after all, they were designed to be bad. Yet, not all of them are equally bad. Pachirisu managed to win the VGC 2014 World Tournament due to its support abilities, which gave big inspiration to all the gimmick Mons users like me. Togedemaru has the very solid Steel typing and good movepool and abilities. Morpeko has OU level power with Aura Wheel, the rest of its traits are underwhelming, but it at least does hit hard. Emolga is not that strong, but it's fast and has 2 immunities. From the Competitive point of view, Plusle and Minun are the worst Electric Mons. They are not entirely bad, but very outclassed by Raichu, their only good strategy, Baton Pass, was banned due to better Mons with the move. Things could have been different if Plus and Minus worked in Single Battles too, as this would give more than 400 Special Attack before any boost. Of course, more offensive movepool would still be needed and the restriction of running the 2 of them (unless you had Plus/Minus Ampharos, Manectric or Klinklang too) is not comfy. In-game, Plusle and Minun are much better, since they learn one of the best moves: Encore. Therefore, in-game the worst Electric has to be the remaining Pikaclone: Dedenne. Not entirely bad Mon either and Cheek Pouch is a fun ability. It only really needs one move to be usable: Moonblast. Why doesn't it learn Moonblast?
Ground: Somehow Grass Wormadam avoided being the worst Grass and the worst Bug. However, Ground Wormadam is not that lucky, unique typing and Quiver Dance isn't enough to compensate all the negative aspects about it, mainly bad Stats and Movepool. I don't really know how to improve this one besides giving more Stats and Movepool.
Fight: Why are there so many similar Fight Mons? Who is worse, Throh or Sawk? Lots of boring Mons here, probably this will be between the above 2, Grapploct and Hitmonchan, which I think is the actual worst (at least Grapploct gets an exclusive trapping move).
Psychic: Easy "win" for Unown. Even if Legends of Arceus buffed Hidden Power, it's still it's only Move and Stats are still bad. Maybe each Unown should learn all moves that Start with its letter. That raises the question about what to do with "?" and "!" Unowns. Give them Midele Power, maybe?
Rock: Ah, yes, the other type with 5 weakness, lots of trash here. Corsola could be the choosen one, but it got Regenerator and enough bulk to use it. Magcargo and Coallosal also avoid being the absolute worst for the same reasons they did in Fire type. Bastiodon has 0 offensive power, but huge bulk and Steel typing. Carbink is entirely outclassed by Diancie, but on its own merits is solid enough, I have actually used this Mon in-game. The one Rock that has almost nothing going for it is Sudowoodo. Sure, it doesn't have Movepool problems and the abilities Sturdy and Rock Head ( with Head Smash, Wood Hammer and Double Edge) are great, but those Stats suck ass. It doesn't hit hard enough for the low bulk and terrible speed it has. It surely needs much more Speed and Grass typing to hit harder with the Wood Hammer.
Ghost: This one is hard, Ghosts are actually good in general. Even the ones that were the worst (Misdreavus and Banette) ended up (Mega)evolving. As weird as it sounds, the worst Ghost has been invented in 8th Gen and has good offensive Stats and wide Movepool: the name is Cursola. Same problem that many of the worst Mons have, low bulk and low Speed. Even Weak Armor isn't enough to outspeed most things, Perish Body meanwhile forces it to switch out. One way to improve Cursola would have been to make Perish Body trap the opponent too. Combined with Protect, this would help to achieve some fun KOs and frustrate the opponent.
Ice: Again, many of the bad Mons here have been improving in recent years: Glalie got Mega, Vanilluxe Snow Warning, Beartic Slush Rush. Dewgong didn't improve much, but bad as it is, the bulk is enough to use Encore both in-game and Double Battles. Delibird on the other hand did get lots of Movepool additions, including Spikes, Destiny Bond and Brave Bird. Stats remain the same, though, so I have to put it as the worst Flying Mon. The most infuriating thing is the fact that it gets Vital Spirit and Insomnia, which are essentially the same ability. If Stats are this bad, why not replace one of those abilities for Huge Power or Wonder Guard? Another option would be to make Present an Ice Move with 250 power.
Dragon: Oh, fuck, this type has been designed to be one of the best, how can I identify the worst one? Turtonator has Shell Smash, Noivern has crazy Speed, Drampa good bulk and Movepool. Altaria could have been the worst, until it got a Mega. Maybe we should look for the one with the worst defensive typing? That has to be Grass Dragon. Of these, Appletun improves the typing with Thick Fat, while Alolan Exeggutor has great power and coverage, along with Harvest, which is always ab plus. This leaves Flapple as the worst. With Hustle, Flapple hits hard, but the problem is (apart from being blind) the lack of coverage, so Steel types wall it entirely. A solution would be giving Flamethrower to Flapple, which would allow it to use the decent Special attack it has.
Steel: Another typing designed to be the best. Fortunately, just like Ground, it has a Steel Wormadam to be a literal piece of trash.
Dark: Yet another strong type. As legends go, Guzzlord is one of the worst, but it's far from being the worst Dark type, that HP is huge. Absol could have been the worst, but even with a wasted opportunity to get the Fairy typing, it at least got a Mega. Mightyena is probably the worst competitively, those Stats are way too low and the offensive moves have low power. Strong Jaw as ability would help it a lot. In-game Mightyena has one of the best abilities in Intimidate, which makes it a good team player. I was about to say Liepard is the worst in-game Dark, but then I remembered it has Prankster + Encore. So, it has to be Thievul. I didn't play SwSh, but from the Stats and movepool perspective, it has few things going for it, every other Dark looks better. Maybe an ability such as Trace would improve it, with more Stats too.
Flying: Technically either Ledian or Delibird are the worst here, but in order to not repeat myself, the next worst has to be Beautifly, outclassed by every single Quiver Dancer.
Fairy: Finally, the end of the list. Dedenne has already been mentioned, so I will spare it here. Slurpuff has Belly Drum and Sticky Web + Unburden, while Aromatisse has Trick Room + huge Wishes. Really, Fairy types are generally by just using Moonblast or Play Rough alone, it's hard to make a choice. Comfey doesn't have Moonblast though and Draining Kiss needs boosts to do actual damage. The bulk leaves a lot to be desired and I don't think Triage is enough, so this will be the worst Fairy.
The period between second and fourth generations looks to be the one in which more lackuster Mons has been introduced. Mons from Kanto have been vastly improved in recent generations, while Mons from BW and afterwards had more work put into them from the very beginning. There are exceptions of course, poor Heatmor is still waiting to be better than his prey Durant.