I think it'd have to be Charmander for me. (Though I pretty much like all three of the Kanto starters equally, probably out of nostalgia.)
Gen 3 and 4 don't stand out for me. I really only ever liked Torchic and Piplup from them. D: And Gen 2 is another where I kind of like all three, but I don't like them as much as Charmander & co.