i know most of the peoples ideas on who it probably is, but my favorite little theory was refreshed in my mind after watching todays anime episode. (although its unlikely) what if tobi was a member of the uzumaki clan from the hidden eddy village i think it was called, as far as sharingan, we have seen countless times that they can be transplanted to non uchiha people, and he could work to achieve the abilities he currently has. My big reason for the idea was when narutos mother mentioned that tobi knew the seal on the 9 tails would be at its weakest during child birth, and i figure that a weakness like that would probably be kept well guarded within a clan. Another reason being that since they were killed off during a past war, no one has really heard anything about them, maybe there were a few rivaling some of the extremely powerful ninja from the other clans, and escaped. I know, not likely, but would be pretty cool
