i level about 80-90 pokemon too level 100 ligit. i was a loser.... it was on crystal the one and only game i played properly. my start pokemon was totidile and that was then my first level 100. it got to the stage were i was beating "RED" just for the xp. =D
Cheating, anything I liked on Red
Legit, Articuno (traded from a legit copy of red or blue at around level 80-90 to help my craptaistic team beat the Pokemon League got the XP on the way back there and through completing it) followed by Rayquaza I've been using since Emerald.
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.