By far, my favorite of the HP books is Prisoner of Azkaban. I feel the story is the tightest of all of them, and with the introduction of James' three friends and their relationship with Snape into the contemporary story, we start to see the history that's shaping Harry now; after all, the events of the series are completely shaped by the events at Hogwarts 50 years ago and 20 years ago. I also feel that it's the last of the good adventure novels. JK excels at lighter single adventures with blindsiding plot twists, and her later books got either too outrageous with its insane machinations (Goblet of Fire), or too serious and talky, which she doesn't do as well.

I haven't read Prisoner of Azkaban in quite a while, so I can't really remember a favorite part of the book.

As far as the movies are concerned, in general, they're nowhere near as good as the books. Philosopher's Stone captured the wonder of the world of magic so well - and it's what drew me into the series - but that was lost in the subsequent films. Prisoner of Azkaban was pretty faithful to the book, and it flowed very well, with its seasonal scene transitions to show the passage of time. But the others, while entertaining, were nothing special. A lot of it has to do with trying to stuff the books into two hours - they really needed to extend the movies or break them into two parts. I just watched Chamber of Secrets last week, and the jerky scene switching left me actually a bit nauseous. And I'll never forgive what they did to Goblet of Fire, gutting Barty Jr.'s backstory (which was one of the only good parts of the book) and making him a one-dimensional psychopath in favor of extending Harry's genius dive-and-snatch into a multiple minute CGI dragon chase (and other fluff).