I have no doubt that this book in the Harry Potter series is the best.
As a hardened reader of fantasy and science fiction I am not often surprised, but this devious and twisty plot was a surprise and a delight. There is also a poignancy about the relationship between the title characters that hits the right note. Coincidentally (or not) the film of the book is also, in my opinion, the best of the films by a long mark.
For me the series reaches a certain height in this third novel that isn't surpassed by the remaining books.