Yeah, I never ruled out Itachi for one minute, but even he was used well in the grand scheme of things and could have been led to believe that dude was Madara the entire time. Nagato could have been explained due to Madara using space time jutsu... even his jutsus hasn't been fully explained, but it wouldn't be a far stretch to guess he could somehow reach into the future in some way.
I'm still betting on Izuna, seeing as they were pretty much equal in all respects and spent most of their childhood's trying to one up each other.
XI Wiki


