Ever since chapter 560 , the chapter where the real Madara Uchiha is shown to be alive. There have been several speculations on the identity of Tobi or " Fake Madara ". My guess is that Fake Madara/Tobi is Izuna Uchiha, the younger brother of Madara Uchiha. I have three pieces of evidence to support my claim.
1.) When Tobi/Fake Madara began to explain the history of the Uchiha clan to Sasuke , the moment when Tobi spoke about the sacrifice of his younger brothers eye as a donation to the clan , The author shows that Tobi clutches his fist , emphasizing the remorse he feels.
2.)In his youth Izuna Uchiha looks extremely similar to Sasuke Uchiha , the path of revenge that Sasuke chose is similar to how Tobi/Fake Madara began his moon eye plan.
3.) Finally , the attack of the nine-tails on Konoha. The nine tails FOX is a fox, and if you were not aware Izuna's name is the name of a FOX spirit. Coincidence?
Well that's all the evidence I have to support my claims , who knows if its true or not. Comment below if you agree or disagree, or even have your own idea on who the Fake Madara really is.