Everyone has theories but only one of them only makes enough sense and that's his brother Izuna. It can't be Madara...you actually have to be dead to be used in the Edo Tensi...that rules him out.
Obito is dead, Kakashi took his eye right...in the battle with Konan it showed that Tobi had both sharingan:
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And to attain the Rinnegan, he had to implant them. Izuna lost both of his eyes. We currently don't have enough info on what powers he actually had. What been said and seen are 2 different things. But search on the forums....there are many theories and I'm not gonna get into a pissing contest on any of those...in my own opinion, I personally think Izuna....no one else would know all the minor details of Madara, no matter how famous of an Uchiha he was...so that's my speculate....