Gen 1: Charmander, but only because Charmeleon is weird looking, at least to me.
Gen 2: Tododile. Same thing, Croconaw ruins it.
Gen 3: Treeko. I honestly love all the Hoenn starters, but Treeko was the one that I had the least fun with.
Gen 4: Chimchar. I really don't like Chimchar for some reason. It's design is just so awkward.
Gen 5: Tepig. Same thing, something about it is just weird. Although, Emboar is pretty cool looking.