Design wise, I think Rhyperior, Lickylicky, Tangrowth, Magmortar, Electivire and (to a lesser extent, mind) Magnezone.
Yup, you read it right. Those are all the Gen IV Pokemon who evolve from Gen I Pokemon. I hate the fact that they added to long-established evolutionary lines with poorly-designed and hideously named abominations. I only slightly dislike Magnezone, however, because I'm playing through Explorers of Sky and I love his flashing police-lights animation. ^o^
I also hate Venomoth because as a child, my 'mons got poisoned by one, and they all fainted, and being young, I cried. And that experience scarred me for life... in hatred of Venomoth!
EDIT: Oh, yeah, and Bidoof!