Tobirama created Edo Tensei for Uchiha Izuna
We all assumed that it was to be Minato, as the other two were the other deceased kage, but a thought I've recently had lead me to believe otherwise.
We all know how Minato died, and when he died. In this time period, it's very unlikely that Orochimaru did not get the information that Minato was sealed away via the Reaper Death Seal, and would know better then to try and reanimate him with the Edo Tensei. If we remember Hiruzen's reaction to seeing the coffin, after seeing the first two were none other then the Senju brothers, it seems very likely and possible that none other then Uchiha Madara was in coffin number three. It was believed that he died at his battle with Hashirama, so Orochimaru probably believed a middle aged Madara was in that coffin when really, it would've been the old Madara we saw in Tobi's reveal.
EDIT: After a couple reminders, I remember Kabuto confirming it was supposed to be Minato. I thought it'd be immature to remove my mistake, so I'm just gonna say read what's under this! Thanks!
Also, instead of making a second thread, I'll simply post my second thought here, along with this one.
As seen in the most recent chapters, Hashirama and Madara seem to have been friends in their youth. Close, even. Whereas Izuna and Tobirama took the ideals of their fathers and were enemies. That last part isn't important, the important part is that Hashirama and Madara were close, and even went against their clans, knowing the other was from the rival clan, to warn the other of the trap.
Putting this with the fact that Tobirama created the forbidden, Edo Tensei Jutsu, one can only ask themselves; Why? Tobirama reminds me of a young Kakashi, someone who works by facts and follows the rules and regulations while ate the same time doing whatever he could for Konoha.
So, after Izuna dies Madara goes berserk. He lost his last, precious little brother and decides to go against those who caused it. I'm guessing he put the blame on either the Uchiha Clan, Hashirama or Tobirama and declared war on that individual. Madara warned Hashirama since Hashi was now the closest person to Madara and Hashi told Tobirama. Tobirama panics. He believes in his strength, but knows he's weaker then Madara (this also would explain why he's so bitter towards the Uchiha) and makes a jutsu that can settle everything. Give Madara his little brother back. The Edo Tensei.