ok im going to tell u who it isnt and reasons why:
Obito:yes obito left eye was givin to kakashi leaving only the right if he was alive.But you do have to remember obito was seen died and the other eye was crushed,also he didnt live in the time period to know half the stuff madara did.And on top of that obito was not a very good ninja or had any special talents besides the sharingan he has.The soppuse it madara actually does have 2 sharingan its simply he wears a mask over the other one, the reason he had to replace the sharingan after the battle with konan is becuase he used izanagi.
zetsu

k this is a tricky one, there isnt alot to know about zetsu cuz....hes not really a person.He is somehow connected to the first hokage cantaining a large amount of his dna. i think i saw some where stating he had like 99.99% of his dna. If zetsu were to be madara it would only be mentally, reason being madara has made it apperent on several occasions that he isnt what he use to be or has some injury that hasnt healed or cant be healed easily.(my guess is that the 4th did a similair jutsu to him as he did with the 9 tails sealing or removing half of his in chakra and half of his yang chakra to some were or someplace, leaving a "shell of himself" behind) that being said he can be using a zetsu body but zetsu himself is not madara.
other notes on madara is that during the uchiha massacre he collectd all of the sharingans there.
+ is able to use rennigan which requeres senjus dna( adding to the possibilty that hes using a zetsu body)
+ can turn intangable which is a yin manipulating jutsu, which is one of the only things ive seen him do
+ has a very old saggy face
+ hasnt been seen using his manguka sharingan