Madara himself. Let me explain that one.
I think we'll see a big epic battle. First it will be Sasuke and Naruto vs. Madara. They'll fight what seems to be an even match, then Madara will get some BS upgrade like a third eye or whatever occular power Kaguya had. Then he'll wipe the floor with them. We'll see some makeshift alliance of Orochimaru & Madara vs. the alliance, while Sasuke and Naruto get back in the fight. In the meantime, Bee and Gai will both die. Kakashi will probably fight Madara and finally die, but in so doing reveal some weakness of Madara's power. I think Madara will be forced to use Izanagi and sacrifice one of his eyes. He'll take Kakashi's as a replacement, probably.
Madara will absorb BZ and take that part of his power back. To be blunt, I think Madara isn't even fighting at half his power right now. Why? He's an arrogant d*** that's why. A lot of his chakra is tied up right now in BZ and has been since before the series began. He used it to keep tabs on Obito. Watch closely how Zetsu and Tobi interact in the first part of part 2. They are slightly antagonistic towards each other. I think also Madara gets the other eye.
IT will eventually go off and rape the alliance. Madara will be forced to deal with the Juubi's full power and eventually succumb to it. I think NB is vastly underestimating Madara's power. In the end Madara will be a victim of his own massive power reserves and arrogance, and that will be what defeats him. I don't think Naruto and Sasuke are capable of defeating Madara. If anyone besides Madara defeats him, it will be the Shinju tree or Kaguya.
