If it isn't Uchiha Madara, then it's either an Uzumaki Clan, probably the first ever jinchuuriki of the Kyuubi? Or the Sage of the Six Paths? Or Obito?
But as of now, I stick to what I've read, someone's theory - it's most probably Uchiha Madara and why Tobi is shocked when he saw who was in it is because he's afraid to reveal his greatest secret, that he's not Uchiha Madara.
Kabuto is great - one of the best character as of now (second to him is the Black Zetsu) ;
He might be an upgrade of Orochimaru, but he's more powerful and bad ass than him. To think that he's almost Immortal is to be considered. Plus, he's testing Oro's jutsu - with matching his own techniques using his own mind. All in all, he's smarter than Oro.