Interesting thread. Let's see how long it takes to jot down my thoughts...
Quality ratio on quantity; the more well liked Pokémon within a generation, the better.
Worst: I think a lot of people will say Gen V, but that gen has a lot of well-liked Pokemon. Chandelure, for instance, is a surprising one. I remember it being hated for being a bad design (much as with Klinklang, Garbodor, and Vanilluxe) but now it seems to be a very popular Pokemon. And there are a lot of well-liked mons from Gen V such as Zoroark, Stoutland, Landorus, Reshiram, Zekrom, Conkeldurr, Krookodile, and everyone's favourite cactus Maractus.
On the contrary, though I think Gen VI on the whole is extremely well-designed, that factor seems to be more of a leveller; very few people's standout favourites seem to be from Gen VI. They're a good bunch of mons, but they're all very middle-of-the-road. Which does not make it the worst or the best.
I'd have to go with Gen VIII as my personal worst. There are some downright awful designs - Stonjourner, Polteageist, and Inteleon come to mind - and very few that stand out as truly inspired. So many of them lean hard into the concept of giving the Pokemon a distinct personality, which actually lessens their uniqueness: all Sobble are timid and cowardly, all Cinderace are braggadocios. I'm not fond of many of the Gen VIII roster, with some exceptions like Appletun and Corviknight.
Best: I do think Gen I has it here. It's the original generation, after all, and if they hadn't been as good as they were most of us might not have stayed with the franchise. The designs are diverse and clever despite being simpler than modern ones, and so many of them hold up today.
How well they aged compared to other generations? Inversively, did they innovate well in comparison to what came before? This is not just concerning the power creep in terms of competitive.
Worst: I think Hoenn's mons actually largely haven't aged that well. This is both statwise - many can't really compete and haven't had the upgrades other Pokemon have (and some, like Sableye and Mawile, have had their upgrades snatched away) - as much as designwise. In the present day, very few of Hoenn's mons are longstanding favourites like other gens are, and I'm not really sure why.
Best: See my answer to the next one for more on this.
How well distributed the type are throughout the group. Was there an oversaturation or drought of a certain type, or was the types well distributed?
Worst: Hate to say it, but it's Gen II for Ghost. Even though Gen I only had one Ghost line, they were given a prominent showing and were made easily accessible. Gen II introduced one sole Ghost-type and locked it off until the very end of the game, even despite there being a Ghost gym. Absolutely awful showing. Gen IV's abysmal Fire representation is a contender too, but Platinum largely redeemed it.
Best: There's a lot of contenders. Hoenn is absolutely swamped with Water-types, particularly in the latter half of the game. Gen V was very good for Bug and Dragon, two types that don't often get a fair showing - Gen V's range of Bug-types in particular is diverse and interesting and widely spread out across the region, with lots of options coming at different points throughout the game, and we unexpectedly got a Bug-specialist Champion! (Yes, I know Alder officially has no specialty, but in BW three of his team are Bug-types, so I've always thought of him as a Bug-type trainer.) It's easy to forget now that we have a lot of competent and powerful Bug-types, but Bug up until Gen V had always been pretty underwhelming, and was one of the "lesser" types imo. Gen IV was decent to it but it's Gen V that really put Bug on the map and made it a type that could properly contend with the others.
How well they are handled in their introductionary generation. Are they introduced properly in a way of most bring reasonably accessable, or were too many of them being out-of-the-way and rare at the same time?
Worst: As poorly as Gen II sometimes handled its roster, I agree with
Bull Of Heaven that Gen IV actually managed to do it worse. Excluding a bunch of the new Pokemon from the regional Pokedex, and not even including Tangela in the game at all! No other game has managed it that badly. Even Platinum's remodel of the Sinnoh Dex didn't fully fix it - even after making room for the rest of the Sinnoh mons, there's still 7 left out.
I'm in two minds about this. On the one hand, there's no reason for them to be there - for instance, Unova includes Landorus and Kyurem in its regional dex despite the fact you can't encounter them until you've got the National Dex. On the other it feels deeply odd and unjust to leave Pokemon out of their region's Pokedex listing. It sort of confirms to me that Sinnoh had too many legendaries and mythicals to fit into the region. But I'll talk more about this in the next section.
Best: The other regions did things varyingly well. While Kanto's roster are all widely available, being the first generation means that it innovated less (and FRLG/LGPE didn't change things too much) - they're all just available wild, or by evolving. Unova has a lot of evolved forms available wild, which makes finding various evolutionary lines easier. Hoenn has a fair few Pokemon which were just as rare as Johto's (1% appearance rates, convoluted methods of access) but people seem to gripe about that less, but on balance I think it struck a good ratio between having to breed, evolve, and hunt out rarer species. I still have a good time ferreting out all the Pokemon in Hoenn.
Were the generation‘s Legendaries and Mythicals handled well, or were too many felt like afterthoughts?
Worst: While I expressed my misgivings about how Sinnoh's legendaries were incorporated, they
were all handled very well, with ample amounts of lore and interesting, unique locations and gimmicks associated with them. So it's hard to criticise when no other generation has given all of its mythicals that much attention and care.
This is why I'm leaving Gen I and II out of the equation - they were less concerned with giving legendaries mythicals special treatment; they were more like bonuses in those gens. III was slightly more interested in them, but still fumbled it by leaving Jirachi out of the games entirely. Even Deoxys didn't really get much explanation or context beyond "it's special, go catch it".
V started the depressing trend of the player simply being given mythicals directly, but it did at least give each of them a decent backstory and associated events (though they're so minimal they barely count). Gen VI wasn't much better. However, I would say VII does it worst of all, not even bothering to give any of its three mythicals any semblance of an event or anything in the way of backstory. Zeraora, Marshadow, and Magearna have virtually no connection and nothing to do with Alola - they're just there. Nor does Zarude to the Isle of Armor, though Gen VII wins the coveted "worst" category for having three mythicals it does nothing with instead of one.
Best: So I think I would, ironically, have to give it to Sinnoh. None of its legendaries or mythicals felt like afterthoughts - all did serve some sort of purpose.
Do the trainers make proper use of many those Pokémon back in their own debuting generation?
Worst: Unfortunately Gen II has it here. So many trainers do not have Pokemon they should have - Morty should have had Misdreavus, Bugsy should have had Pineco, Ledyba, or Yanma, Will should have had Slowking, Karen should have had Tyranitar. No trainer uses Octillery, or Ampharos, or Scizor. And so on. These arguments have been made before, and they're all glaring oversights. Gen II wins hands down.
Best: Gen IV, by deliberate design. Because the player has to see all the Pokemon to complete the dex (not catch), every Pokemon in the game is used by at least one trainer. This means that there is a wide variety in the enemy rosters (particularly in Platinum) and strongly encourages exploration. I really liked this model, and while I think it'd get pretty tiresome if it were the case in every game it works for DPP.
