Okay, there are two reasonable answers of Tobi's identity.
1.Madara Uchiha
2.Izuna Uchiha
Reasonable thinking:
It CAN only be someone who has many years of experience and old age. When you see Tobi take his mask off when Zetsu was with him you see he has liver spots, wrinkles, ect. So it CANT be Obito. But it MAY be true that Madara found Obito and used his body by using Zetsu's first Hokage cells. Another reason why it CANT be Obito is the lack of motivation of going bad. Why would a good, kind hearted, heck of a ninja just go bad??? It MAY be true that it is indeed Obito's body but Madara. You may complain that Tobi has the same sharingan power as Kakashi, which has Obito's sharingan,but Madara and Izuna were the strongest of the Uchia, don't you think that they would already master a amateur sharingan of a 12-13 year old boy???
Madara Uchiha is one reasonable candidate because of his experience and knowledge of the ninja world. When Tobi was fighting Minato, Minato noticed that the "masked man" can control the nine tails. The battle between Madara and Hashirama is already out, you see him summon nine-tails. Only the most elite of Uchiha can do this, which is only Madara and Izuna. The only person that was on Madara's level was his twin brother Izuna.
Remember, Madara took Izuna's eyes, later on "Tobi" declares when he was fighting Konan that he was just taking back what was his, this basically meaning that the true identity of Tobi must be a descendant of the Sage of Six Paths, the true wielder of the Rinnengan. If you read the manga Madara unlocked the power of the Rinnengan after he fought Hashirama. Escaping with Hashirama's DNA. Therefore creating Zetsu. He could have used Zetsu's special ability to form himself with Obito's corpse. Which is why it MIGHT be true that Tobi is a part of Madara Uchiha and Zetsu together.Remember Tobi, or Madara declared that he was just a shadow of his former self, that he was incomplete. This meaning he is not whole! Which is very LIKELY why Tobi and Madara are at two different spots. When Kabutomaru showed what was in the coffin Tobi was really interested something that was VERY important to him. If you think its Madara it could be his true body, so he can be whole. If you believe its Izuna, the one in the coffin can be his brother, Madara, which is already identified that it was indeed Madara in it, in the latest chapter.
Tobi CAN be Izuna because, as I said, when Tobi was fighting Konan he declared he's just taking back whats his. Madara did take Izuna's eyes. When Madara unlocked the Rinnengan he gave it to Nagato, convincing him to make the Akatsuki. MAYBE Izuna's trying to get his eyes back. Madara and Izuna were always known as equals and even in EVERYWAY meaning it is VERY POSSIBLE that they have the same abilities.
IM NOT SURE WHO IT IS. BUT I WANT TO KNOW RIGHT NOW!!!!!!!!!!!!!!!!>:3
MY GUESS, HE'S MADARA UCHIHA