Goblet of Fire, mainly for directorial and editorial reasons. Scenes were jumbled and cut in odd places, and quite frankly it didn't seem to flow as easily as it should have. I wished we could have seen more of the Quidditch World Cup. Secondly, I have to say Chamber of Secrets, which is odd since when it first came out I saw it several times in the theater and rewatched it over and over again. However, when looking at it compared to the other films, it really does lag on quite a bit, though the second book isn't exactly as action-packed or as interesting as the rest of the books tend to be, so perhaps that's why. I don't hate it, it's just not my favorite of the films/books as a whole.
Although I will say that COS did introduce us to the lovely Jason Isaacs who portrays the lusciously delicious Lucius Malfoy. I mean come on, how can you hate that? ^_^
Rest of the 30 Days of Harry Potter Challenge.