The Order of the Phoenix is the worst. It is also unjustifiably the longest. The plot just doesn't come together like it does in the other books. A lot of it felt like filler and angst.
Edit: I mean Grawp! What is the point? And they fly to the ministry on thestrals? It takes Harry a year to remember what door he was looking at? Kreacher is a terrible character. And even I can't believe Harry would believe that Voldemort cornered Sirius Black, the two of them have next to no contact with each other for the entire series. I stopped reading that book for about 2 weeks the first time I picked it up. I still "like" it, but it is definitely the weakest.
http://www.youtube.com/watch?v=F1gWECYYOSo
Please Watch/Share this video so it gets shown in Hollywood.