I was so disappointed by Magnezone's design that I can't use a magnemite anymore. Same with Rhyperior, Tangrowth, Togekiss, Magmortar, and Porygon Z.
Gen 4 was pretty terrible. Almost all of the evolutions of previous pokemon looked horrible in comparison to the original it evolved from. I mean, Gallade is cool. Same with Froslass, Dusknoir, Mamoswine, and Gliscor, but holy crap the rest suck!
http://i.imgur.com/xWWX5xm.jpg http://i.imgur.com/tMhFpCD.jpg Thanks Fellwolf and L4DHunter! <3
Rhyperior and Magmortar are the first two that come to mind. Add Tangrowth and Lickilicky to that, but I've never used a Tangela or Lickitung anyway.
I don't love Togekiss's design, but I don't hate it, either, so meh, I'll use it. I don't like Magnezone's design, but my fondness for its typing, stats, and evolutionary line are too great to ruin it for me.
If forms count, then Newtwo is coming pretty close.
I honestly think Megnaton should of had Magnezone's design, Porygon-Z is one of my favorites, And while I don't care enough to actually use it... I can't bring myself to hate Probopass. Gotta love a Pokemon with a nice mustache.