Being the girl as I am, I'm a fan of the cute pokemon.
If I were to dress up as one I'd be pikachu but my favorite has always been jigglypuff if we're talking red/blue version.
If we're going a little past that, the cutest pokemon ever (cutest = my favorite) would be
Igglybuff.
Even if it is pathetic and can only gaze at it's opponent. lol