Here you have the worst Pokémon of each Gen:
- Gen I: Tauros
- Gen II: Raikou
- Gen III: Salamence
- Gen IV: Garchomp
- Gen V: Volcarona
- Gen VI: Aegislash
- Gen VII: Tapu Koko

For Gen VIII I don't know who's the worst but so far it seems that the worst Pokémon is:

[/spoiler:lit]Dragapult