My first level 100 was my Empoleon my second will be Shaymin ;492;.
I might wait untill I get pokemon platinum before I get my Shaymin ;492-s; to level 100.
I actually know someone who got the missingno. pokemon with a master ball, the tried levelling it up past level zero with rare candies, but gave up and saved and turned off her silver version, then she had limitless master balls and rare candies. She leveled up poke's to lv.100 all the time but they were weaker than normal Lv.100's.
My first Level 100 was Charizard, the starter I used for FireRed when I first got it <3 I loved that little guy. He was my favorite xD But unfortunately, when I was forced to restart Diamond I didn't realize I didn't transfer him over... R.I.P. T_T
The first ever level 100 I had was Blastoise in Pokémon blue but my first level 100 I actually leveled up without exploiting the missingno glitch was Raichu from the same game.
Mine was my Absol, Aiu, from my Ruby version. It would have been my Zangoose, Shiyaa, but the game I had him in was stolen ._.
I also had a level 100 Lugia, but I caught it at 70, so I don't count it.