I think it could possibly be Jiriaya...I say that because of that single scene, where he barely taps naruto's head, and sends him flying off a cliff from the middle of a forest...what power....and that's only a taste huh? I dont know if that was just precise chakra control or what, but that was a bad ass tap. Of course, it could be Oro... I dont think it was the fourth, although he succeeded in sealing Kyuubi, I dont think that is the same as defeating him, since he was only sealed. In any case, I guess we'll find out by watching more anime!!!