I'm actually currently watching the film series for the very first time (and I've never bothered with the books) at the rate of one a weekend. Watched Sorcerer's Stone and Chamber of Secrets so far, both of which were very "meh." I actually had watched those two years ago when they first came out on DVD, and they didn't exactly convince me to want to watch any future installments. But judging from what I'm reading here, I'm looking forward to watching Prisoner of Azkaban this weekend now.
I thought PoA was the first truly great Potter movie, and it remains my favorite to this day. Alfonso Cuaron just did so many things to change the films for the better. I'd probably rank the movies thusly: 1. Prisoner of Azkaban 2. Order of the Phoenix 3. Deathly Hallows, Part II 4. Deathly Hallows, Part I 5. Half-Blood Prince 6. Goblet of Fire 7. Chamber of Secrets 8. Sorcerer's Stone
About Prisoner of Azkhaban, I certainly agree Alfonso Cauron made an excellent movie, and he made a good choice with the tone. However, I think people are a little unfair in believing it was all Alfonso. The way I look at it, the first two movies/books, are when Harry is new to the Magical World. All the small feats of magic are front and center with a spotlight on them, the colors are bright and vivid, the streets and the walls of the castle are pristine, etc, etc...because we're seeing the world through Harry's eyes, and it's all new to him, he's young and Naive (11 and 12 Years old). By the Thrid movie, the colors are toned down, the little feats of Magic are moved into the background, the streets are dirtier, the castle walls are marred from 1000 years of underage magical abuse. This all, to me, represents that Harry is now more familiar and acclimated with the Magical World, it's no longer all a big amazing wonder, he no longer naively sees things so perfect, he can see the details now. The transition from the first two movies/books, is similar to when you move into a new house or apartment, you think it all looks so fresh and new at first, and after you've been there for awhile, you see that crack on the base board, or ugly cupboard handles, or the paint glitch in the corner of the living room, etc. So, like I said, I don't take anything away from Alfonso's movie, he did a great job, and Columbus, very well may have screwed it up, and kept it in the same tone as the first two, but, I do believe the text of the third book neccesitated this change in tone.
I think Cuaron would have made better movies out of any of the seven he didn't direct, regardless of their source material's tone or place in the story.
I'm pretty sure it was all Cuaron's doing that so drastically changed the visual look of the movie. Columbus made clean bright fake looking movies, and Cuaron dirtied it up and made it look real, a trend the future directors decided to carry on.
The books also take that "grittier" not so fantastic world and no longer naive outlook turn with the 3rd book.
PoA is my favourite book - even if it doesn't really move the plot forward as much as I had hoped. Having said that, there were brilliant scenes with both Sirius and Remus, but essentially removing the climax of the book (the whole Shrieking Shack-revelations) for the sake of that ridiculous werewolf scene was just a bit too much to bear for me. My vote goes to OotP - I found the book way too long, so I appreciated the shortness of the movie (IIRC it was the shortest movie which is kind of strange given that the book was the longest one...). And I really liked Luna, the Harry-Sirius scenes, the possession scene. My ranking: 1. OotP 2. DH, II 3. GoF 4. CoS . . . . 5. DH, I 6. PS - because it introduced me to the Potter-series (read the first book prior to seeing the movie, and immediately afterwards read the 2nd through to the 4th) 7. PoA 8. HbP