I don't want to die slowly, I don't want to decay; I want to be chosen, I want to be made
I'd have to say Riku. Even if it was just the two of them sans any powers, I think Riku would still win, just because he's probably physically stronger than Roxas, and much more skilled at fighting.
I don't want to die lonely and weary of life; I will not be earth bound, I'm goin' to fly