Gen I: Voltorb line, when it comes to creative these Pokemon are the best and they are the most Pokémon looking uh... Pokémon.
Gen II: Sunflora. Terrible and tragic, design wise and stat wise. Wh
Gen III: Exploud and Loudred. If I were looking for the definition of disgusting, they are definitely the top choices.
Gen IV: It's a tie between Lickilicky and Probopass. Horrendous designs.
Gen V: Out of all the Pokemon I have listed, these two really take the cake Simipour and Oshawott (along two others from Gen 7, namely Brionne and Popplio). Uninspiring. While Simipour looks like a ripoff character from the Donkey Kong series, actually Oshawott looks original. However, that doesn't make it less ugly. Fortunately, both Dewott and Samurott look great!
Gen VI: With only around 70 then-new Pokémon, it isn't that difficult to spot for the fugly ones. The Slurpuff line aren't too pleasant to the eyes, Slurpuff itself is the most terrible out of the evolution family. It looks like a cross between an old hag and a dog.
Gen 7: Drampa, Turtonator, Blacephalon and Stakataka. Just no, especially the latter two, they look like they were designed by the children of Game Freak employees. Brionne and Popplio, they look like the long lost basement sisters of Oshawott. The Jangmo-o line, I'd like to think that they existed just to make the unfortunate basement sisters look pretty, while they failed their mission, they are still ugly, regardless.