Harry Potter and the Half-Blood Prince

Essential, and it carries the single most load-bearing scene in the series: the explanation of what a Horcrux is and why there are several. The final two films are a treasure hunt that makes no sense without it, and this is the only place it is spelled out.

It is also, oddly, the funniest of the eight. Yates spends a surprising amount of a very dark film on teenagers mishandling their feelings, there is an extended sequence about a love potion that plays as pure farce, and Jim Broadbent's Slughorn is a comic creation of real quality, a flattering, frightened man who has spent fifty years collecting useful students.

Bruno Delbonnel photographed it and it is the best-looking film in the series by a distance, nearly monochrome, lit by candles and firelight, with a desaturation so aggressive that the studio reportedly queried it. The cave sequence near the end is the most frightening thing in any of them.

The complaint is that it cuts the wrong things. Most of the flashbacks explaining Voldemort's history are dropped, which is the material the film is supposedly about, and the final battle at Hogwarts from the book is removed entirely, so the ending is quiet in a way that feels like a budget rather than a choice. Watch it anyway; the next two depend on it absolutely.