Clefable. With nothing but Metronome. I loved, and love, Metronome more than any other move.
Of course, that's not counting the many level 150 or so Pokémon I've caught with the Missingno. trick, and the dozens of Pokémon brought to level 100 with [][][] M []'s sweet bounty of rare candies.
My first level 255 was a level 81 Kangaskhan, just evolved from [][][] M []. I traded the Kangaskhan over to Pokémon gold and used in battle, which caused it to go all the way to level 255, then down to 0, then up to level 63.
When I caught a wild Mew, I levelled it up to level 100 in about 1 second with the experience underflow glitch. My fastest level 100, though not the first one I got (experience underflow was discovered fairly recently).