The Avengers

Essential, and it is the film that proved the entire premise. Five separate franchises converging into one picture had never been attempted, and the industry spent the following decade trying to reproduce it.

What makes it work is that most of the running time is people arguing on a flying ship. Joss Whedon's structure gives every pair a scene together before anything is resolved: Stark and Banner in a lab, Thor and Loki on a cliff, Rogers and Stark failing to like each other, Romanoff interrogating a god by pretending to be rattled. By the time they are a team, the audience has watched them refuse to be one for ninety minutes.

The Manhattan sequence invented the modern third act, for better and worse. The long apparently unbroken shot that moves between all six heroes across the battle is still the clearest statement of what this franchise was for.

It has aged in specific places. The Chitauri are anonymous, the plot is a glowing cube in a tower, and Hawkeye spends most of the film mind-controlled. None of that matters much. Watch it after the five films before it, which is the only time the franchise's homework has ever been straightforwardly worth doing.