I'm gonna say Snape. He's not hugely powerful, but he knows the important stuff. I mena, he folled not only Voldemort, but also practically the entire Hogwarts, by killing Dumbledore. I think Harry could have learned a great deal from him if it wasn't for Snape's past with Lily and James.

Yes, Dumbledore is powerful, no doubt. But there were a lot of things we couldn't do - like what Snape did.