OK. This has been explained many many times, but Tobi CANNOT be Obito, the timeline does not match, there is no motive, and the looks themselves don't match (from what we've seen, Tobi looks quite old when he should be Kakashi's age). Also, any way for him to have escaped is COMPLETE bull, why would Zetsu save an Uchiha who no longer have a functioning sharingan from death? Kakashi was the one with the sharingan, should have gone for him....
Tobi is NOT Obito.
Moving on.
I was wondering about Kagami, could somebody link me the page in the manga where he appears? Remember, if he does not appear in the manga itself, it can never be him.
The Shisui theory shouldn't be true for reasons like the Obito one, mainly because of the age problem, where Shisui should be just a little older than Itachi, but Tobi is clearly far older. Also, Shisui's dead body was supposedly found and whatnot, but there is the strange fact that his body was not used in edo tensei...The Shisui theory is better than the Obito one, but it's still one of the least possible ones.
The elder brother just seems too far-fetched because there seems to be no way he could live that long.
The Zetsu clone of Madara doesn't work in my opinion because of Tobi's space-time techniques, so far Madara hasn't shown anything of the kind, and he has had plenty of opportunities to, which leads me to believe he can't, and so Tobi shouldn't be some clone thing unless Madara shows space-time abilities. And if he does, then this one becomes one of the most likely for sure.
In reality, all these theories are hard to find any basis. All we really know is that Madara was planning this from since he was alive, so the person has to be old. Which leads me to believe it is either just some other guy called Tobi with impressive abilities, or Izuna/an Izuna-Zetsu reincarnation. Izuna was shown in his coffin, but there is a possibility that that was a fake (like Kisame's). The timeline fits, the old-ness fits, the motive could easily fit, and the fact that there are new techniques fit. So Izuna is in my opinion one of the better theories. Which just goes to show how bad they all are...