I was really unsure how Order of the Phoenix would turn out as a movie, as like most people I find the book to be one of the weaker entries of the series.
However to my utter astonishment , Order of the Phoenix ends up being one of the best films of the series. People will complain about the movie being too short for being based on the longest book, but in my opinion there was very little that got cut which I felt should have been in the movie. One of the biggest flaws of the book is it drags a lot in places, and the film does an excellent job of trimming certain scenes down. The relationship between Harry and Sirius is one of the main highlights of the film, Evanna Lynch is terrific as the bizarre but loveable Luna Lovegood and the climax battle in the Department of Mysteries is some of the best work of the entire franchise.
The only real flaw I have with this film is how it fails to develop the character of Ginny Weasley compared to the book. True, she is given a lot of screen time in the film, but she barely has any dialogue so she often comes across as an extra rather than a main character. However, Bonnie does a very good job with what she is given.
In conclusion, Order of the Phoenix is a great entry in the series and certainly one of the most underrated.