Style-wise: Dark Riku > Duel Wielding Roxas > Aqua > the rest.
Gameplay? I haven't played days yet, and I don't really count CoM, so that just leaves Sora, Aqua, Ventus, and Terra. So from just that list, my preference goes: Aqua > Sora = Ventus > Terra. I do like all of them, but Terra's a bit slow for my taste. Aqua's powerful magic, acrobatic motion, variety of elements, and ability to teleport really puts her out in front for me. As for Sora and Ventus, They're pretty similar, imo.