In a duel, I wouldn't be surprised if Kouyoku takes it. Dude's a beast of a fighter (heck he was very impressive as far back as the time of the Coalition - and he has obviously been improving since then). Not many people are going to best current Kouyoku 1vs1.
In a war with equalized forces, based on what we've seen until now, Gyou’Un. That being said sooner or later Kouyoku is going to be above people like Gyou’Un.