Okay waiting for hate on my opinion haha.
Gen 1- Jynx. I was always weirded out by it's face even as kid. Never liked it at all.
Gen 2- A tie between Sunflora and Remoraid. Sunflora's face is freaky some reason and Remoraid is a really ugly fish similar to Magikarp.
Gen 3- Lileep. Cradily isn't much better either.
Gen 4- Purugly- Ugly is even in it's name too. Shot for awful pun.
Gen 5- Either Druddigon , Cryogonal, Vullaby, or Roggenrola's line.
I won't say for Gen 6 until all the new Pokemon are revealed so I can't fully judge.