Back in the old days (With only the original 151) I think mine was Bulbasaur, can't remember exactly.
Today It's a toss up (Damn my immature mind taking that the wrong way :L) between Gliscor, Electivire, Aipom (And to a lesser extent Ambipom), Bulbasaur, Flareon and Espeon, oh and Lugia.
Then again there's Munchax, Togekiss, Torterra )It's a freakin' Tank), and a while back I quite liked Corsola, got one to level 100 back on Rby for no real reason other than I loved it xD
Then again, I likez Mudkipz too. :L
Thats the lowest I can put it down to I swear, I could name soo many more that I like.