Because he was the first pokemon I ever got to level 100, and he's just all around badass.
Welcome to the club, BROTHA!
I also like Pikachu, Snivy and Zoroark. Pikachu, because who doesn't (that probably isn't in the double digits of age yet)? Snivy for putting Mudkip in its place, and Zororark, who SHOULD have been playable in SSB4.