My first level 100 pokemon was Meganium in Pokemon Crystal, followed shortly by Tyranitar - I just kept fighting the pokemon league, got huge amounts of experience. Those two have so far been my ONLY level 100's.
Strangely, Meganium isn't my favourite Johto starter - Typhlosion is, but I already had Typhosion in Gold. (Typhlosion is also my favourite starter of all time - followed by Charizard, Tyranitar is my favourite non-legendary pokemon overall).