I think he is either Izuna, or he really is Madara just in a another body, notice how Itachi and Tobi say that "he's a shell of his former self" And Tobi says he's not as strong as he was back then, "I'm just a mere shell of my former self." He's definitely not Kagami, Obito, The Sage Of Six Paths, nor his sons, Shisui, or Teuchi(ramen shop owner lol)
In Naruto Manga series, Tobi is one of the main antagonists of the series who, among other things, played an integral role in the formation of Akatsuki. He also was responsible for the Nine-Tails' Attack on Konoha. Get more information from http://naruto.wikia.com/wiki/Tobi.