My favorite Pok�mon because of looks has always been Gengar, but I've never played with him in my deck so... The two Pok�mon I do always get are Vaporeon and Umbreon. So I guess a tie between those two. And I was 11 when I saw the first movie with Mew and Mewtwo, so they hold a special fucking awesome place in my heart.
On a side-note I wish they would release a Pok�mon game where you can get all Pok�mon, and raise the available Pok�mon on hand. Like a free-roaming world with all maps and gyms. They won't do it because they're bent on two versions/capture restrictions every time because of the money it makes, but still...
Umbreon is my favorite and has been for quite a while. I initially saw it when I played Pokemon Colosseum so many years ago (it was one of the first Pokemon games I played) and immediately fell in love with its design.