After the revelation of the 6th coffin , another theory has been started. I myself can't decide who really is behind the mask of Tobi. At first I think that he was Obito. And after reading some threads I think he really is Uchiha Madara . Whether Obito or Madara let us wait until Kishi reveals his face. But first let us see the reasons why Tobi is Uchiha Madara. And it is up for you to decide who really is Tobi.
Tobi is Uchiha Madara
Remember Tobi said that he gave Nagato the rinnegan. The rinnegan would be used to revive Uchiha Madara's body. Uchiha Madara that came out in the 6th coffin said " So, it finally happened. It would appear that little brat Nagato has managed to grow."
But how is it possible that there are two Uchiha Madara? (from the 6th coffin and Tobi). Here's the theory. Uchiha Madara used the DNA of the 1st hokage to create a clone of himself like the zetsus. The zetsu is able to retain the memory of the original body. You have seen this in the 4th world ninja war when Neji was cloned by a zetsu. Tobi also said " I'm just a clone of my former self. "
Other reasons why Tobi is Uchiha Madara - Well, I am no Sherlock Holmes. Just sharing what I've thought of.
1. The 4th Hokage warned Naruto that " He's the most dangerous villain Naruto will ever face". Obito is a clown and couldn't do much.
2. In the manga chapter, Tobi clearly said that " I am Uchiha Madara "
3. Obito's mangekyou sharingan may control space and time? like Kakashi. The flashback from Kisame depicted Uchiha Madara 's appearance not Obito. Uchiha Madara as Tobi have controlled the 4th Mizukage and has controlled the Kyubi for the 2nd time. Something that Obito may be can't do.
4. Tobi's mask has one hole on the right. Remember Obito's death? He gave his right eye to Kakashi. But Tobi's battle with Konan reveals two sharingan of Tobi.
5. Itachi planted amaterasu on Sasuke's eyes. If it is Obito, it won't be triggered since that was made especially for Madara only.
6. Tobi , how could he know much?
Tobi is not Obito! Tobi fought the fourth Hokage. We all knew that the fourth is Tobi's sensei and Kakashi was still a teen at that time. So Obito should also be at the same age as Kakashi.
Also read: Kakashi: The Copy Ninja Unmasked! and Mystery behind Tobi's Mask.