It's hard not to pick skitty if you've seen it in the anime.
If I were to go by the sprites alone, however, I would have to go with mew's Ruby/Sapphire sprite or azurill's pre-HGSS sprite. I also personally love torchic since I have a canary of a similar color that is just as cute
Mijumaru, hands down. Before 5th Gen, it had to be Piplup. im infatuated with the cuteness of the Water Starters, from Squirtle to Totodile, and Mudkip too, but Gible is another one that very adorable ^_^