There's not too many to choose from, but for me, it's gotta be Clefairy! The cutest, most elusive, ex-normal type of all time!
So what makes me like Clefairy? Well, Clefairy Says, for one. Pokémon Stadium had some good minigames, but that one was my favourite by far. (Way better than the Ekans ring toss, anyway.) I also used a Clefairy in Leaf Green, once (and evolved her into a Clefable), and she was SO GOOD that I took to calling her "Super Clefable". Yeah. Also Bill is a Clefairy and I think that's awesome.
Level 100: No Clefairy, but my Clefable, ANGELIQUE (OT: CRYSTAL), from one of my many Leaf Green runs.
Runner ups: Azumarill, Sylveon, Gardevoir.