i say that it is the sage of six path...there was a hint that kabuto gave us"dont worry i didnt tell anyone else" i doubt that would be madara's bro,no one cares about madara's bro. could be the first cause he was famous but that would be a shock to leaf cause they knew orochimaru did it once so i thought it has to be someone who is super famous and thats the sage of six path the hint that i wrote above would be most fitting to the sage because it was said that he was a legend it would be him that will shock everyone.madara even said "how did you do this" and that kabuto was a"madman"..so give it a thought which could be so shocking to madara that he says that kabuto is a madman. and obviously that person is strong enough to make madara forcefully accept kabuto..to me its no other than the sage himself.
If Kabuto did summon the Sage of Six paths, that would tie in well with the story:
The sage of six paths was the Juubi host, and Madara wants to become the Juubi host as a part of his moons-eye plan.
As you said, Madara called Kabuto a "madman", which makes the theory seem a little more possible. But even then, Kabuto would still need the soul of the SO6P to do that technique - if no-one knows what he looked like, how hard do you think it would be to find his soul in the afterlife?
I'm not sure - maybe Kabuto had help from Orochimaru or something like that, but I'm not entirely sure he could find SO6P and summon him on his own. That just seems ridiculously awesome - even for Kabuto.
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Kabuto could have summoned the first Hokage:
I seem to remember that Madara is only a shell of his former self - no where near as powerful as he used to be. With that in mind, the reincarnation of his old enemy would surely put him into shock:
The first hokage defeat Madara when he was at the height of his power, so he could easily defeat Madara now - even if he was just a reincarnation.
Another thing to consider is that Orochimaru was already able to use this summon, so Kabuto doing it is not far fetched and is entirely plausible.
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Kabuto may have even summoned Madara's brother:
Madara's brother probably wouldn't be a match for Madara, but still, he is his brother - maybe Madara just didn't want to fight against his own kin.
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Finally, does anyone think that maybe Kabuto summoned someone we don't already know about?
Naruto's world is a big place - there could be stacks of powerful ninja we have never heard of.
Killerbee is a good example - we only just found out about him recently and already it seems like he is one of the most powerful ninja in the entire series.
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It seems there are too many possibilities to narrow it down at this point.