Either Alakazam, Feraligatr, Lanturn or Ampharos.
I think it was Ampharos, the other three came quickly thereafter. At the end of my 500 hour pokémon Gold game I had a whole box filled with level 100s. Those four were part of my main party, however.
Along with Mewtwo and Venusaur I destroyed everyone in Pokémon Stadium 2.
I never reached level 100 in Pokémon Blue or Yellow, mind you. There weren't enough things to keep me busy.