Gen II: Dunsparce. While I know that it's supposed to be based on te tsuchinoko, its face doesn't look anything like a snake, IMO.
Gen III: Slaking. I mean, just look at it.
Gen IV: Rhyperior. IMO, Rhydon was fine where it was.
Gen V: Tie between Garbodor and Druddigon
Come on, Rhyperior and Pinsir look cool. Agreed with Dunsparce, but I still think Raikou is uglier. Slaking is ok. Druddigon is ok. Garbodor... totally agreed.