Tobi is not Madara Uchiha and that is my verdict.

1. Zetsu’s Approach

Zetsu keeps the name Tobi and he is the only person who is very close to Tobi, since he is a spy and has the most information in the entire Naruto universe, he exists everywhere planted beforehand blended with the environment.
Zetsu also wonders how much Kabuto knows about them, in vary tone. They’ve got something to hide other than their fighting powers. Kabuto already knows most of their secrets which they’ve got hidden even from Akatsuki members to avoid an internal clash for power.

2. Tobi’s awe of Kabuto’s Re-animation.


Okay, this is very very intriguing. Madara Uchiha is afraid of something; it’s also a very best kept secret. since Kabuto promises that he hasn't told a soul. Madara who can analyze and formulate any jutsu asks “how did you do this?”
Who is he afraid of, let us think back, He is not afraid of anyone. He has had only reverence for Hashirama, even then it cannot be first or the second Hokages since they were sealed by the third Hokage during the battle with Orochimaru.
Its might not be the fourth Hokage either because even Minato agrees that Madara’s technique is superior to both his and the seconds, also Madara has become even stronger after that, he has had Hashirama’s power embedded into him, He also has the Rinnegan now. So it cannot be the fourth Hokage.
So we can conclude that the person/corpse which is shown to Tobi is either the sage of six paths (?!?) or Madara Uchiha himself. While Tobi is not Madara uchiha but someone from the uchiha clan’s missing nin, or someone who took over an aging Madara Uchiha while he was exiled and took the sharingans out of it and over time grew stronger because of the potent cells in him.
This theory also has many loose ends such as Tobi would not have access to Hashirama’s cells to create a clone. Madara Uchiha of the Mangekyuo is not a simpleton to be just overtaken by someone without a mangekyuo or even a sharingan. And If Itachi himself believed that Tobi was Madara then Tobi must be quite talented to satisfy Itachi and make him believe.
But we’ll soon see, I guess.

3. Too much similarity between Tobi and Obito.
Uses only the right sharingan and even while using Izanagi he sacrificed his left sharingan, just like Danzo sacrificing the sharingans in his hand, like a mere tool. So the right sharingan is not his. If its not his, then what happened to it? He might have donated it to Kakashi while he posed to die in that rock slide by using his body warping technique.

Hmmmm.. guess……….