Even though I don't like Cao Cao very much, after getting a bit into the actual history, not the "modified" version of Luo Guanzhong, I have to admit that he was a very skilled leader and politician.
Liu Bei may have been a more kind ruler, but in the end he did lack some qualities a man needs to unify a country. And even if Cao Cao and his successors were tyrants, they managed to end an era of nearly constant war, which deserves respect.
And lastly he did never let the world betray him.