It seems like the better the book, the worse the film.

Goblet Of Fire was one of the best books in the series, and it got terrible treatment in it's adaptation.

Order Of The Phoenix was a pretty weak book untill the last little bit, and it's movie was really good.

I think Prisoner Of Azkaban was the most faithful, though I would've liked Sirrius to have been as menacing a threat as he was in the book. Gary Oldman played it flawlessly, but many great scenes were cut that added to the fear.

I like Philosopher's/Sorcerer's Stone the best, simply because it is the most visually interesting. It's also more magical, where as the others really aren't. It seems more like Dragon Ball Z than wizardry in the later films.