Captain America: Civil War

Essential, and it is an Avengers film in everything but name. Watch The Winter Soldier and Age of Ultron first: the entire argument rests on what happened in both, and the emotional load is carried by a friendship established across three films.

The achievement is that both sides are right. A set of accords placing the team under oversight is proposed, Stark agrees because he has been personally confronted by a mother whose son he killed, and Rogers refuses because he has watched an institution he trusted turn out to be the enemy. Neither position is strawmanned and the film never resolves the question.

The airport sequence is the best large-scale action Marvel has staged, and it works because nobody is trying to kill anybody, which makes it legible and sad. Spider-Man and Black Panther are both introduced in it and both are immediately better than their trailers suggested.

The last twenty minutes are what the film is actually for. The conflict collapses from policy into a personal act of violence in a bunker, with a villain whose entire plan was to show one man a video, and the fight that follows is three people who love each other doing permanent damage. Daniel Brühl's Zemo is the best-conceived antagonist in the Infinity Saga.