At first I absolutely considered Tobito true as well.
Kishi has just thrown in so many suggestions:
1.)TOBI =OBIT / OBITO = TOBIO => take the O from TOBIO and it results in TOBI and OBITO......also the thing of saying Obito really fast over and again
2.)Only the right Sharingan eye
The only problem I currently have with the Tobito theory is the fact that Obito was still a child when he died, meaning that his body was of a child as well.
Also half of Obito´s body was crushed by that rock, including his face, and if he was reconstructed with Zetsu parts not only should Tobi then be much smaller but he should also have a partial Zetsu face, but we saw his lower face line when he removed his mask.
One thing that gets me thinking about this is the fact that Kakashi´s (Obito´s) Kamui and Tobi´s S/T look very much alike, I think Tobi is just using a further enhanced Kamui.
Basically, I believe that Tobi has something to do with Obito´s "supposedly crushed" eye, but I do not think that he is indeed Obito.
Because Obito died as a child, shows a grown up voice, fully adult human face, and is also fully grown, and even with Zetsu parts transplanted some parts of his body should then still be the ones of a child.
Just my own theory though, Tobi is just the hardest thing to crack, but there has to be a reason why Kishi has made all this Obito Tobi hints.