Tobi, who he is or is not:
1) Not Madara (literal)...Madara id dead
2) Not Madara's brother...Wouldn't need Zetsu Parts
3) Not Obito...Age is wrong
4) NOt Sasuke's dad...Itachi killed him
5) Not a non Uchiha...His knowledge of Uchiha secretes, plus his Sharingan abilities rule this out
I think the answer is becoming more obvious and hard to deny. Tobi is a special Zetsu clone, infused with Madara's chakra (Recognized by the Kyuubi), Infused with Madara's memories, Infused with Madara's Will. He may or may not have Madara's DNA.
Open to question is whether or not he is still loyal to the original Madara.