Probably the best of the Harry Potter books to this point. It is considerably darker than the previous books and everyone is starting to notice the opposite sex. That was absolutely necessary to keep the books fresh at all. Even still, the book can be a little too formulaic. I don't see the need for the Dursleys at this point.
I think that if you look at the reason for the very great success of the Potter books it is that Rowling's prose is just so accessible and easy to read and the books flow very well. It never drags even though this is the fourth go around on what is hardly ground breaking material.