Swampert. When I played through Sapphire when I was seven I tried to beat the Elite Four with only my Swampert, which I did. The thing is a tankkk haha.
To list a few : Pikachu (obviously, lol), Celebi, Espeon, Blastoise, Mew, Togepi, Umbreon, Lugia, Swampert, Snorlax... and I just love them all, pretty much. Well, I've never played Black or White, so I like most of the 493 that were made before B&W. I may play Black or White sometime, but so far, most of the new Pokemon look kind of dumb imho.