The first one probably doesn't count, as it was an accident. It was a Weepinbell. I dropped my Pokemon Red into a puddle of water. My mother helped me get it back to working. When I popped it into my Gameboy Color, I went looking through my pc and found a level 98 Bellsprout sitting in there and was like "WTF!?!?!?!"
The first time I hit level 100, without glitching or anything, was with a Pidgeot on my Pokemon Gold.