Gen I: Pinsir; it really creeped me out when I first saw it.
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