Mine is easy. Buizel. And what's funny about it is I think 4th gen was the worst generation (Before plat fixed all their mistakes, but that doesn't mean they didn't happen) But I still love buizels more then any other pokemon.
I stopped playing Colosseum just because I like to finish the games, but I didn't want to evolve it. So I got stuck between my likeness for makuhita and wanting to complete the game and evolving all of the pokemon.