I didn't really feel like posting in this, but since I don't like how most list threads are deprived of explanations of the choices, I figured I might as well set an example if I want things to improve.
Generation 1
Best:
Kadabra's got it all. A spoon, a dope moustache, a big-ass wasp-butt and a crotch-vent. They don't come cooler than this, people.
Worst:
Electrode is the embodiment of lazy filler Pokémon. While Voltorb had a simple design, it was to complement the game itself. A pokémon disguised as an item. A Pokémon mimic. But I guess they decided they need more Pokémon and just made Voltorb bigger, changed his eyes and switched the colours in place. As lazy as it is uncreative and terrible.
Generation 2
Best:
I like elephants and Donphan delivers on being an elephant. Apparently he can also curl up into a ball or something, according to Google, and while this is cool, I never really saw this in the games. I just think he looks rad. It's a shame he appeared so late into the game.
Worst:
Hello, Pikachu! ...oh, you're not Pikachu? Oh, you're one of those new low-effort baby Pokémon. I'll just come out an say it. With the exception of maybe Elekid, I like zero of the baby Pokémon designs. Pichu is by far the worst one, since every time I see it, I can't help but think "This Pokémon already existed".
Generation 3
Best:
Ludicolo's a pineappleduck with a poncho and a sombrero. What's not to like?
Worst:
Pichus? Again? Goddamn it, go home! Oh, they're not only NOT Pichu, they also for some reason are two separate creatures. You know, it's actually kind of impressive. Looking at this generation, I thought it would be incredibly difficult to find the absolute worst one. It's almost exclusively trash (especially the Regi ones), yet for some reason it was super easy anyway to settle on Plusle and Minun. This has to be the creative low point in the history of designing.
Generation 4
Best:
Garchomp has too many colours and spikes for my liking, but it's a solid Pokémon.
Worst:
This was probably the most difficult round to pick from. While it doesn't have as many awful designs as the third one, most of the bad ones are equally terrible. This gen introduced a whole new lot of baby Pokémon and classics like Balloon, Honeycomb, another Regi and Swirly Energy Thing. But then someone had the brilliant idea to poison previous generations of Pokémon some more, so they gave cool Pokémon that couldn't evolve before, newer uglier forms. However someone out there got ballsy and wanted to see what could happen if they gave a really shitty Pokémon the same treatment, and thus Probopass was born. The Pokémon with Nosepasses for hands, a LEGO stud for a hat and a beard.
Generation 5
Best:
This gen was surprisingly decent for the most part. It took me a while to warm up to Eelektross, but once I accepted him in my team, I couldn't go back. This giant electric lamprey has no weaknesses because for some reason it can levitate.
Worst:
There aren't a lot of stinkers in this pack, but you can tell they were running out of ideas and needed to fill some slots with random things they could see out their window. You have multiple Pokémon that are just a bunch of gears, multiple ice cream cone ones and for some reason three near identical legendaries. The one that bothered me most though was Scrafty. It's a gross lizard thing that partially moulted its skin off from its previous form, but still wears it as a hoodie and baggy trousers.