I think it's Madara's body. That might be the reason why Madara seems to be impervious or immune to attacks. And why he needed Sasuke to kill the-wannabe-hokage (forgot his name) because he can't do it by himself. Have we even seen Madara deal any damage so far? All i can remember is him teleporting himself and others and having all attacks pass right though himself.

If it's not his body i would guess it's his Brother. Can't see how The Forth would make such an impact on Madara. And i refuse to even consider the possibility of Obito. He is dead and has no link what-so-ever to Madara (and if it would turn out to be him then i am 100% sure that Kubo has taken over Naruto).