It always depends on the situation. You can have the greatest general on the offensive, but loose in the end to logistics *cough*germany*cough*. So it would have to be an allrounder general. I like Cao Cao, he is like a Chinese Napoleon - great in administrative, cultural and military things. There isn't a single thing that he didn't reform during his reign. If you go purely by K/D then I guess someone like admiral Yi could win.