When it comes to 1v1 duelling Ouki was a monster but in that I'd have to go with Moubu, extreme diff. Moubu was so devastating that he managed to best Kanmei, who himself had a claim to being the strongest fighter in China and whom Hara had previously martially hyped by feeding him the most physically imposing fighter of the old Qin Six members. Moubu looked like an absolute berseker, that source of strenght was insane.
Man'U is himself a martial monster, in fact we are all hyped because of the showing he had against Moubu, but I think that in a duel to the death I'd still go with Moubu.
1v1 fight I'd go Moubu >= Kanmei > Ouki, with Ouki pushing either of the giants to extreme diff.