Maybe i'm alone with this, but i enjoyed playing Xaldin and Riku the most. After that, DW Roxas, Zexion and maybe Xemnas, i guess. Lexaeus is ridiculously broken, whilst almost everyone else dishes out pretty low damage.
I like to play as Larxene, Roxas, Saïx and Xemnas. So in layman's terms, I like to play as my favourite Org. XIII members. I've tried out a few of the other members, but haven't had as much success playing as them.