I know that that this has been discussed THOUSANDS of times in the past, to the point where it's overrated. However, based on the events and scenes we are shown, the question comes up once again: Could Tobi (Madara) be Obito?
Obviously, Madara is a separate entity that lived his own life at one point. However, it was never fully explained what happened after his body was weakened. Madara was able to live for well over 80 years, but his prestige and strength as one of the founders of the Leaf and his being rival to Hashirama were used to cover this fact. Truth is, Hashirama whom was supposed to have inherited the Sage's "body" and life force, still died at one point. How is it that Madara lived after being damaged so badly, that even Hashirama assumed he died?
One possibility (maybe a stretch) could be that near his death he transferred into a different body, similar to Orochimaru's techniques. With this concept, Madara could keep transferring whenever near death, explaining why Madara could age over 80 years.
But, this theory in itself has many flaws. It might explain why Madara doesn't look like such an old man, but why would Madara choose the body of an extremely damaged Uchiha such as Obito? It explains the hair and the right eye, but it still doesn't explain why Madara would have chosen Obito's body.
That aside, the main fact that stuck out to me was that Kakashi could not use his Mangekyou Sharingan on Madara. Some people may have believed that his mastery of the Sharingan is the explanation, to the point where he may be impervious to it. But how is it that Sasuke's (or rather Itachi's) Sharingan had an effect Madara? Amaterasu clearly damaged him and Madara even admitted that if he had taught Itachi more about the Sharingan, Itachi could have killed him. Meaning that Itachi's Sharingan was still a threat to Madara, while Kakashi's is harmless.
The explanation to this may be that if Madara is using Obito's body and Sharingan, Kakashi's Sharingan (oringinally Obito's) might not work on its original owner.