I like how you said advisors, and not strategists, first of all.
I would have to go with Guo Jia, as he was one of the major reasons Yuan Shao was defeated. Xun Yu was great as well, but Cao Cao lacked trust in him, as all his other advisors. Heck, we could all be saying Guo Jia was best simply because Cao Cao favored him. That's what happens when you have so many advisors. Jia Xu was great, but we only truly see his greatness when he against Cao Cao. Therefore, while he's probably the best advisor out of the group, he wasn't Cao Cao's best.
Huang Zhong: Are you calling me old?!
Wei Yan: ...Yes.
Huang Zhong: Liu Bei! I demand you kill him!
Liu Bei: Well it's... true...