Guan Yu
Pei Songzhi commented on this as follows: "Cao Cao admired Guan Yu's character even though he knew that Guan Yu would not remain under him. He did not send his men to pursue Guan Yu when Guan Yu left, so as to allow Guan Yu to fulfil his allegiance (to Liu Bei). If he was not as magnanimous as a great warlord should be, how would he allow this to happen? This was an example of Cao Cao's goodness."