To me the fifth film was the pinnacle of the series. Whereas many of the other films captured the events of the book but not the spirit, the fifth succeeded on its own as a movie, independent of the novel, by capturing a somber, brooding, and melancholy atmosphere that was present in the book. It was beautiful and very well-acted, I loved it.
I saw Bruno last night instead (tickets were sold out here for the midnight showings), though I'm not sure how excited I am about this one. The sixth novel was a crushing disappointment for me, as I felt the Harry-Ginny relationship was contrived and Rowling's prose worsened in quality with it. However, the book's ending did make up for many of those missteps, and it may be interesting to see on film.