Well yea' the most Realistic theories are Izuna Uchiha and Tobi-Madara Uchiha split clone/baby or whatever.
But yea' Split clobe/baby version is quite good too, lets think about it..
Madara had everything over the control, befpre he died he splited, and made a second weakend version of him self to end a job for him.. And Madara gave Nagato the Rinnengan, just for the purpose that in future when the job is done madara's split part would just Revive him self, and something like that..
So as I say, we can believe 2 theories.
1. Izuna Uchiha.
2. Tobi - Madara Uchiha split part.
All Obito stories are Bullshit.
Why ? Obito had only 1 Eye left, and Tobi had 2.. Can't remember ? When he fights the konan he uses Izanagi, for that he sacrifises 1 of his eye it means he had 2, anyways Shisui version is fcuk''d up too, Itachi killed him, we know that for sure.