I guess I'll go on record as saying I think its Izuna. Now here comes the TL;DR stuff you'll all skip.
The story sort of mirrors itself, so the age order is different, but the rest holds true. Itachi is the new Izuna, Sasuke is the new Madara, Naruto is the new Hashirama.
Izuna Uchiha was a harmonic and devoted person. Itachi is a harmonic and devoted person.
Both gave up their eyes to their brothers willingly. Both do horrible things that they believe to be for the greater good (moon eye plan, uchiha massacre).
Madara seeks power. Sasuke seeks power. Sasuke's forgiveness or defeat will end the cycle of hatred that Madara started.
Hashirama tamed the tailed beasts. Naruto will befriend the tailed beasts.
Besides those sort of story telling devices, there's the elimination of other suspects, most of whom were too young to have been Tobi at the time of Naruto's birth.
Beyond that, there's Tobi's insistence that Izuna's eyes were given willingly, narrowing the field down to the two who would know for sure, either Izuna or Madara. From there, the "other Madara" thing blowing Tobi's cover as Madara leads me to believe he's Izuna. Without the larger than life reputation, he's "no one".