My first level 100 was all the way back in Pokemon Yellow, the starter Pikachu. I've always loved electric types and until Luxray came along my favorite was Raichu. I attribute this to the fact that Pikachu in Yellow can't evolve and that was the game I had, so Raichu was like this unattainable awesome beast that I wished I could get. To compensate for being unable to get a Raichu, though, I went through and completely overleveled my Pikachu so it'd be just as good. Then I got Gold, had my Gameboy stolen (thankfully my Yellow cart wasn't in it, although my Gold cart was so that went bye bye), and now my Yellow cart doesn't work so my Pikachu is long gone. Sad day.
Past that Pikachu, I don't usually get Pokemon to level 100. I like to play through the Elite 4 and I hate just walking through them, so I try to keep my team around the 65-70 range. Usually a few in the lower end and a few in the higher end. With that said, I do have one level 100 in X with another on the way. The one that's at level 100 is my Mewtwo. I used it for leveling my second team, so it got to level 100 pretty quick. The other one that's on its way is my Talonflame. It's my helper for breeding, doing all the flying and it has flame body to help hatch eggs. When I'm off catching stuff or EV training a Pokemon, I typically have Talonflame with me so at this point it's up to level 80ish and will continue to rise. I don't intend to retire it any time soon so it'll get there. xD