I was just about to create a thread for this; I think it's some one very close to Tobi or so call Madara; my first guess was it was Madara's body but if that the case he would'nt be able to bring is soul back because it's stll there.
Edotensai binds the dead persons soul to a sacrificed individual body, so if Tobi is Madara; the sixth is coffin therefore cannot be Madara.
with the information we have at the moment it has to be his brother, but when you look at the conversation Tobi and Kabuto had hinted that Tobi is not who he say he is hmmm.....