Probably Prisoner of Azkaban as I believe it's the one that remains truest to the source material, a few of the lines in the Shrieking Shack were even verbatum apart from a misplaced word or two, and has one of the precious rare positive additions; the moment when Snape throws himself between werewolf Lupin and the trio.

The dementors also have the best presence here of their three film appearances, their animation and vibes were phenominal.