Tobi is the ten tails....
It bothers to see some responses to this theory so here I go with my points....
1.Tobi sharingan is always active. The only time we see this is when the user is NOT an uchiha.....ex. kakasi and danzo's are always active.
2.Tobi has a room full of sharingan. Obviously there are only two reasons to have such a ting. One reason is your gonna use izanagi a thousand times....or the second and more likely is that your experimenting on how to perfect the sharingan and learning how to cmbine powers and strengths. For example....taking the space/time jutsu from one eye and taking the powerful genjutsu of another.
3. Madara refers to eye of moon plan as "our plan" if he cloned himself to accomplish something he wud have said "my plan".
4. While we're on madara..let's tink about this. Madara fought with hashirama countless times, the villages made peace, madara left, then came back to get some of hashirama dna......THINK ABOUT IT.!! Mdara had plenty of chances to get that dna prior to that fight. No, after madara left someone told him about the power derived frm senju and uchiha power combined. This was tobi. In exchange for this info and senju dna infusion afterward, madara gave tobi senju dna. Also, madara never hted uchiha.....if he wanted uchiha dead he wud have killed many b4 he left. And when he came back he only went for senju BECAUSE of surpressing uchiha.
5. Madara knew about nagato...but how cud he give nagato the rinnegan wen u got it just b4 death. No, tobi told madara how to get rinnegan, madara went and got senju dna, then after alittle expirementing tobi was able to create it in madara. By his time madara was old and injured, so tobi convinced him to give up his remaining power in exchange for revival later. Madara agreed and tobi then used research gathred to infuse nagato in secret. (Tobi controlled hidden mist at the time thru 4th mizukag).
6. Tobi durability is outstanding....but notice he only comes out of the shadows after madara dies. Why.? If he's not a madara clone...here's why. Tobi is a black zetsu created by the sage of six paths. Let me start with madara. White zetsu was made from hashirama dna. Dna tobi got AFTER madara fight with 1st hokage. So prior to this black zetsu was a whole, and prolly not too powerful because madara was the first uchiha to amass his level of power. All tobi (black zetsu) had was knowledge. But thru deals with madara tobi acquired not only senju cells, but a deceased uchiha body. By splitting himself in half, tobi (or black zetsu) was able to attatch his left half to madara's body and his right half to an "alter ego" of hashirama senju. He probably had madara help him with making white zetsu and attaching his right half first. Then aftr madara died zetsu attached the left half to madara, forming tobi.
7. Assuming tobi originally was just a black zetsu with extensive knowledge, prior to meeting madara....where did tis black zetsu come from. I belive it was the sage. Every tailed beast has a mind and personality. So what of the ten tails mind.? The sage seperated the ten tails chakra into the tailed beast and locked its body in a "stone prison". But where did he put its mind. The sage cud create from nothing and I believe he created a body for the ten tails and put its mind inside. Maybe as a type of second chance. I think the sage could have locked this body away in a pocket dimension which someone later stumbled into. Releasng the
ten tals in its human form. (The trapped in a pocket dimension thing is unlikely, but maybe)
8. The elder son of the sage was def the ten tails (tobi's) first victim. Whether he was released by the son, or was never trapped and was roaming free...he somehow got the sons ear. The ten tails thrived on hatred and he turned the elder son against the youner son. This is something the ten tail wud do over and over again. Ex. Madara vs. Hashirama and sasuke vs naruto.
9. As a "nobody" in a body created by the sage probably with whatever chakra was leftover from the creation of bijuus tobi only goal was to undo what his enemy the sage had done to him. The only trick is he needed the sage to undo it. So instead he wud use the sage's descendants. He wud play them against each other, all the while acquiring more power. The ten tails is the enemy of humanity.
10. The ten tails body and eye of the moon plan. The sage is a myth "based" on fact. He did not create the moon. So where is this "stone prison". THINK ABOUT IT.!!! The rinnegan (which only the sage had) comes with many summons. We know the sage could create from nothing so its obvious he created these beasts....but only one stand apart from the rest. The demonic statue. Bound by chains with a conveniant bandage over its face with 9 eyes for the tailed beast. Think about it. Sage created the tailed beast b4 his death, why wud he have a summoning to seal them.??? Think about it, when he created the tailed beast he let them run free. So what was this thing prior to the creation of the bijuus. This is the stone prison.!!! This statue houses the mindlles and chakra-less ten tails body. Which is why it had to drain nagato clean dry b4 it could attack Hanzo.
11.Uchiha massacre...this is simple. Once nagato had grown and tobi (thru mizukage) orchestrated hanzo to push nagato off the edge he had a rinnegan and no longer needed the uchiha. It seems since tobi is not a real persn he cannot create nor improve on jutsu. He needs someone else to progress then use their power for himself. So tobi attacks konoha with kyubii and relies on danzo to order uchiha massacre. After which he and danzo (maybe seperately) collected all the sharingan left behind.
12. Which uchiha is tobi.? Over time there have been many powerfl warriors, andi belive tobi has had a sample of most of their power. When tobi realizes danzo has shisui's eye he goes bananas trying to get it. This is because tobi is a collector not a creator. He takes power from evryone he can trying to become stronger. I belive his pocket dimension jutsu came from the elder son of sage. The elder son eyes had a spiral pattern, one that resembles the pattern of tobi's original mask. Now tobis mask resembles the ten tails eye. I think madara, obito, shisui, and many other uchiha have had there power added to tobi's power pool through various and unknown means.
So there it is....tobi is a zetsu created by the sage with the mind of the ten tails. All he knows is hatred and darkness and wishes to plague mankind as he did once before. The only man to outsmart him was the sage....and naruto who seems to be resembling the sage more and more will be his final opponent.
P.s. I may have left some points out....let me know what u think. I mixed facts with theories.