Order of the Phoenix
Hm, not bad, not particularly exciting, either. PoA and GoF were better. It's the longest but also the most disjointed book and the last quarter seems rather rushed.
At least we know more about Harry's and Voldemort's history.
It's a bit like
The Empire Strikes Back, things happen, stuff is explained but there is no definite end (V not defeated or even weakened, he just lost a couple of his followers), with the difference that Empire is the best SW film.
( Spoilers, just in case somebody still hasn't read it )