Yeah, its been made lots of times but i wanted to do a rundown:
I'll give points for and against everyone so you can decide for yourself.
So far, the candidates thought to possibly be Tobi are:
1. Madara clone/copy
6. The 1st Uchiha
7. Fugaku Uchiha
If we analyze them:
1. Madara clone/copy:
Tobi has vast knowledge, on many things such as history of the uchiha, their feud with the Senju since the founding of Konoha that only Madara could have had. Madara was the only known uchiha who had controlled the Kyuubi like Tobi did.
Madara was revived via IWR. This technique cannot work if the soul of the person summoned is in the impure world(this world), where Tobi was. Plot wise it also seems to destroy the whole point of him being brought back if Tobi=Madara.
Kakashi's MS which was originally taken from Obito, has a jutsu that looks similar to the one Tobi uses.And Tobi kept his right eye uncovered, while Kakashi took the left eye.
Obito would have no reason to take over the world and could'nt have gained so much power so fast. -.-
Was around at the time of the 2nd hokage, so he could have learned space time from him. He was also associated with the elders who have a lot of knowledge on Konoha, etc.
He's basically almost filler and has no role outside the flashback of Danzo so he does'nt fit in the plot.
He was Madara's brother, no one would know more about how to imitate him. He was likely to have read the uchiha stone tablet and learned many things with his Mangekyuo Sharingan.
Izuna's body was shown dead in the coffin. He also lost his eyes when Madara attained EMS.
His name has been used many times by Kishi, in the 5 kage arc and when Itachi's gift to Naruto is revealed. His genjutsu could control others w/o them noticing, like Danzo controlled Mifune, so its possible when Tobi controlled the Mizukage the same/similar tech was used
Shisui was said to have drowned in part 1. One eye was shown in Danzo's possesion, the other in Itachi's.
6.The 1st Uchiha:
He was the elder son of the Sage of the 6 paths, so no one would have as much knowledge as him and this may explain how Tobi knows so much about the paths of the rikkudo.
He lived a very long time ago, outdating the ninja world itself and should'nt be alive after so much time. His face is also unknown to everyone, so no one will recognize him even if Tobi takes off his mask(plot hole).
He was absent during the night of the kyuubi attack, as said by Itachi and may have been the orchestrator. It would also explain why Tobi never condemned Itachi even after being betrayed by him, and why he's always protective of Sasuke, not handing him over to Kabuto even after seeing Edo Madara.
Against: Fugaku is not likely to have learnt so much about the Kyuubi and the history of the Sage, etc. He was also shown to be killed by Itachi.
Your decision, me i'm still undecided.