Harry Potter and the Prisoner of Azkaban

Essential, and the best of the eight. Alfonso Cuarón took over and changed everything that could be changed: the camera moves, the uniforms are worn badly, the castle is in a different landscape, and the children behave like thirteen-year-olds rather than like characters in an illustration.

The most quietly radical thing he did was weather. The film is organised around the turning year, with long dissolves through the Whomping Willow as it loses and regains its leaves, and the effect is that time is passing in a place rather than events happening in a plot. Almost nothing in the first two films suggests anyone lives at Hogwarts. This one is about living there.

It is the only entry with no Voldemort in it, which is why it can afford to be about something else: a boy who has been told almost nothing about his parents finding out what the adults around him were like at his age. The Dementors are the best creature design in the series and the film treats them as depression rather than as monsters.

Michael Gambon replaced Richard Harris, who died in 2002, and plays a noticeably more restless Dumbledore. The time-travel sequence in the last act is the tightest piece of plotting in the series and is the one bit of the book that the film genuinely improves. If somebody will only watch one of these, this is the one.