I'd have said Phanpy, if not for the fact that my first encounter with the creature involved my eardrums being punished by recycled 'Phanpy PHAN! PHANPAAAAY!'s.
There's also the fact that it evolved, but hey. I needed something to be negative about, right?
Answering the topic at hand; Bayleef. Torkoal also had it's moments.
I like pretty much all of Ash's portrayed-as-cute Pokémon like Pikachu, Snorunt, Cyndaquil and Squirtle [etc.], but I think Corphish is the most adorable-looking one of them all.
Although Totodile is hella cute as well, Corphish stays number one. Especially when it wears that headband. It's wuvliness <3