Harry Potter and the Order of the Phoenix may have cut a lot from the original book, and it is the first of the films to have a much darker tone, but it still has a lot going for it.
Considering how much was in the book it became an amazing film adaption, and due to these five reasons, quite possibly the best of the series.
1. Harry is at his best
The movies obviously focus almost continuously on Harry throughout. Yet this film shows more of Harry's state of mind, giving the audience a better understanding of the franchise's leading character. As well as Daniel Radcliffe showing a definite refinement in his acting ability, this movie emphasizes more than the others Harry's feelings of loneliness, confusion, and regret.
2. The new characters are perfectly cast
OotP sees a bunch of new characters introduced into the franchise. Including Luna Lovegood (Evanna Lynch), Nymphadora Tonks (Natalia Tena), and Dolores Umbridge (Imelda Staunton). Each fitted perfectly into their new roles, making it seem like no one else could have possibly ever played those parts. Imelda Staunton was so good she is now often listed as one of movies most hated characters.
3. Balances the serious side with the fun side well
Cedric's death in the Goblet of Fire finale sets a much darker tone for the next movie. The most memorable moments of Order of the Phoenix include Umbridge's sadistically cruel corporal punishments, Harry's struggling PTSD after the events in the graveyard, and of course Sirius' death. Yet the film counters the darkness with some well-timed gags from the Weasley twins and that awkward/amusing Cho kissing scene.
4. The friendships
Each year at Hogwarts there is always some tension within the friend group. Especially the year before with all of the teenage angst in GoF. Yet the friendships seem stronger than ever throughout this film. Leading to some genuinely heart-warming moments between the friends as they form Dumbledore's Army. There are also some touching scenes between Harry and Sirius, especially from Christmas at Grimmauld Place.
5. Voldemort's connection with Harry
Although Voldemort (Ralph Fiennes) 'officially' entered the series at the end of the previous movie, we start to see more of his villainous character at play in this one. Also, all of the subtle signs show Harry's increasing and troubling connection with the Dark Lord. These indications may be short, but they add a real feeling of danger for the first time in the franchise.