Based solely on physical strength, Ou Ki, aka the Monstrous Bird of Qin, is easily one of the strongest characters (if not the strongest). He was skilled in both offence and defence, possessed tremendous willpower and mental fortitude, and his mere presence could leave his opponents shaking in their boots. He was also the most skilled with the glaive, which he passed down to his student, Shin.
As a Great General of the Heavens and one of the Six Great Generals of the first generation, Ou Ki was a well-seasoned commander and most of his past battles weren’t portrayed in the anime. In the current timeline, however, he demonstrated his combat abilities during Sei Kyou’s Rebellion, the Keiyou Campaign, and, most notably, the Battle of Bayou. His leadership is unprecedented, as he could boost morale simply by being visible, and it took two of the most brutal Zhao officers to kill him. And even then, his death came as a result of underhanded methods.