I believe Moubu and Kanmei beat Ouki in a duel but there's plenty of reasons to think otherwise. It's a tough one.
I also give the edge to Kanmei/Moubu against Ouki strictly in a 1vs1 duel but Ouki's indvidual martial hype was pretty ridiculous.
- Saitaku mentioned Ouki in the vein of being stronger than Moubu, who was calling himself the strongest in China. "Strongest in China" rings an Ouki bell, at least in Saitaku.
- Ouki's performance against Houken, where he was on the brink of killing him a few times (and got interrupted). It's worth noticing that Houken had given Bayou Moubu the "trembling arm" thing when they had briefly seen each other. Overall, I'm pretty positive that Bayou Moubu wouldn't have won against Ouki.
Like I said I favor Kanmei/Moubu in a duel. But it would be an insanely hard match. If someone has the stance of Ouki beating them, it's certainly not a ridiculous one to have.