Gotta say Roxas simply due to the fact that he wields two keyblade without a form gauge. Sorry sora, you can't use two keyblades, your not wearing the right clothes.
Why do I feel a yoda moment coming on. Watch the next kingdom hearts, he going to pull out a keyblade and start flipping around and stuff. Actually I'd like to see that, that would be hilarious.
Also, there is speculation that kairi is indeed a keyblade wielder, cause she comes in contact with aqua's keyblade in BBS.