Glameow vs Purugly? come on, no one? Well, the thread is new I guess. I was never particularly fond of glameow, but it evolves into something so ugly, that ugly has become a part of its name!
Also, I am glad though that we got Samurott out of the way. It's not terrible looking I guess, but Dewott was just really cool looking. And now for the list:
Eggscute - Eggsectutor (or however their names are spelled)
Trubbish - Garbador (went from ugly to fiendishly ugly)
Pichu - Pikachu - Raichu (progressively less cute with each evolution. Still, I do think Raichu is still kind of cute, but it's kind of like that child that was really cute at 4 and below, but now is 6-7 and still trying to throw its cutesyness around when it just isn't quite the same and it becomes a bit awkward)
Squirtle and Bulbasaur looked best at their second stages
Dratini - Dragonair - Dragonite (from cute majestic blue dragons, to giant yellow stubby dragon)
Treeko - Grovyle - Sceptile (Grovyle was IMO the only cool looking one out of the bunch)
Rattatta - Raticate (ugh, my goodness what a terrible thing evolution did to that cute little bastard)