Probably the best one I've seen so far.
The first flight of Buckbeak is absolutely iconic, as is Harry's Patronus, probably the best spell cast in the movies that I've seen.
It still has some of the goofiness of the first two, but it's pared down to a pretty manageable degree. All the kid actors have finally developed into... actors. They do a really good job with the material. Also, this one feels like it earns it's length more than the other two.
Of the films this feels the most experimental. Alfonso is pulling out jazz. It's more interested in themes than plot, which it resolves with a pretty poorly integrated, if fun, McGuffin.