Over the years my favorite Harry Potter movie has changed numerous times. For a while, my favorite Harry Potter movie was just the latest one. As I got a little older, I began to constantly describe Prisoner of Azkaban as my favorite film because I found myself watching it almost every week (compared to every month or so). I still absolutely love Prisoner of Azkaban and consider it one of my favorites, but over the past few years it's been over shadowed by the last three films.
Even though Prisoner of Azkaban was my favorite, it's also the one I complain about the most. I mean, where's the Marauder's backstory!? I think that's the reason it slipped lower on the list. I love the way the last three films were done, and there weren't any major changes that I got really upset about. If the Marauder's story was in PoA it would probably still be my top Harry Potter film.
Out of the last three, I think Deathly Hallows Part II is currently my favorite. I say currently because I don't feel like I've seen it enough to firmly proclaim it as my favorite. I only saw it twice in theaters, and I definitely need to see it more before saying it's my absolute favorite. From my two viewings, I think it'll be able to stay at the top after a million and more viewings. I thought the whole movie was beautiful and brilliant. Were things cut and changed? Yes, but none of it was done in a way that ruined the story. Were things added? Yes, but I thought they did it really well. Things were different, but it was still very true to the story J.K. Rowling created.
Although I don't want to inscribe it in concrete yet, I'm fairly confident that Deathly Hallows Part II will be my favorite Harry Potter movie for years to come. (But I want to say that I love them all so much, and it's very hard to choose between them.)