Well right now the only Organization members we really know are the ones from Chain of Memories (unless you imported a Japanese copy of the game I guess). And of those, Lexaeus and Zexion are only from Reverse/Rebirth. Based on character, my favorite is Axel. He's crazy, but also a little mysterious. We don't really know (in COM) what his motives are. Zexion also intrigues me, but he seems to be the most "dark" out of them all. He, like Axel, uses whoever he thinks he can benefit from.
Just based on appearances, I'd say Saix seems pretty interesting, but I'll have to wait to play the game to find out more about him.