The only one I've gotten to 100 was Mewtwo and that was solely through the DP Elite Four, leveling up Pokemon to evolve them and fill the Pokedex. Since it was an import from Leaf Green, Mewtwo was getting an EXP boost. My second highest was I think Charizard at level 80-something. I don't EV train so my Mewtwo's not as powerful as anything else out there. I don't care though (not like I know how to EV train anyway).

EDIT: Just looked in my Diamond version. Charizard's on 89.