Whereas the first two HP films were deliberately whimsical, if still entertaining, The Prisoner of Azkaban introduced a less than subtle hint that something darker and really quite unpleasant was looming on the horizon, a bit of a logical progression given that we start off with the protagonists being 11, young kids, and as they get older things get progressively more difficult and complicated. Sure, everybody is aware (I think) that the Harry Potter series is basically an analogy for growing up (as well as lots of other things, eg Voldemort's "halfblood" obsession being a not-so-suble reference to the Third Reich), but not many people give it credit for just how clever it is.

In any case, The Goblet of Fire took that progression one step further: it had the quirky, whimsical elements that made its predecessors so charming but also an ending that is SO dark and pessimistic that it was unprecedented even to those who'd read the book. As such it's probably the best of the series