Captain America: The Winter Soldier

Essential, and the best film in the first three phases. It is also the one with the largest consequences: it dissolves the organisation that had been holding the franchise together, which changes the premise of everything after it.

The Russo brothers made it as a 1970s conspiracy thriller, explicitly, down to casting Robert Redford as the senior official. The subject is pre-emption: an agency proposing to identify and kill people before they have done anything, defended in a boardroom with reasonable arguments, and the film's hero objecting on principle to something his own side has built. That is not a metaphor the genre usually risks.

The action is the other half of why it matters. The lift sequence, the causeway ambush and the fight with the Winter Soldier are close-quarters, heavily practical and cut widely enough to follow, and they reset the standard for every Marvel film afterwards.

Sebastian Stan's Bucky is given almost no dialogue and carries the emotional weight entirely through recognition. Anthony Mackie's Falcon arrives fully formed. The last twenty minutes dump a great deal of classified material onto the internet, which the franchise then had to live with.