Nothing in canon says Harry is one of the greatest of wizards. He has his talents, but he's a classic example of the "Average Guy as Hero." This is a hard trope to pull off because readers keep wanting him to be super-powerful, but the whole point is that he's not extraordinary. He's ordinary like the reader is ordinary, but he's required to be a hero and somehow he manages to be one. That implies that we ordinary readers could be heroic too, if it were necessary. That connection is spoiled if Harry is a great wizard in a league with Dumbledore.