Madara. Going by the recent showcasing of Madara's abilities and that Oni video there.
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There really isn't much footage of what Oni is capable of. All we know is that he's much stronger than Akuma, who can already split islands without a problem. He also has a move called the Shun Goku Satsu... [url]http://streetfighter.wikia.com/wiki/Raging_Demon[/url]
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Yeah, still Madara then since he has all kinds of abilities to stay away from physical attacks, though his eye techniques would be what saves him if Oni is so strong since the Raikage and Tsunade were able to do damage to his Susano'o therefore it would only provide him with temporary protection. From the sound of things too, Oni doesn't seem massively intelligent and would have no knowledge of genjutsu right? So it'd probably be easy for madara to put him under an illusion and finish him.