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Gen I: Nothing I actually hate in this gen. If I absolutely have to pick one it's Farfetch'd because, while I like it stylistically and wish it could evolve into something useful, it is literally designed with the express purpose of being a useless unevolvable joke.
Gen II: Unown. It's a hilariously useless piece of shit designed to be a one-species collection sidequest, and it's unbreedable so you can only complete the set by waiting through endless goddamn random encounters for that one final letter. And the rewards for completing the set range from nothing to damn near nothing depending on the game.
Gen III: Delcatty. It is the single weakest Pokémon you need to use an evo stone to get (and one of the weakest fully evolved Pokémon of any kind) and it has what in almost all respects is a HINDERING ability. Also it's ugly and boring.
Gen IV: Phione. So we already got a legendary that's only obtainable (to this day!) from a specific side game, but let's add this weird thing you get by breeding it with Ditto. Not only is Phione very similar to Manaphy visually, it is in every respect nothing but an inferior version of it. There is not a single move Phione gets that Manaphy doesn't, not a single stat in which Phione is even slightly better. Phione doesn't even have any lore attached to it. WHAT IS THE POINT?
This one gets a special shout out for being my least favorite Pokémon in the entire franchise.
Gen V: Gotta be the elemental monkeys, as a group. Bland, ugly copymons that BW1 practically forces you to use in the early game. If I have to single out one it's Simisear, for being the most redundant (it's an inferior Infernape in stats and movepool even if you ignore it being a monkey) and for literally having shit in its design.
Gen VI: Sylveon, for being the only actual evolution of a previous-gen mon and evolving by Amie, which not only nothing else does but leads to weird fridge logic about Amie vs. friendship with Umbreon and Espeon.