MCU Films to Watch Before Avengers: Infinity War

Short answer: All of them. The payoff at the close of Infinity War will be greatly enhanced by having seen all the movies released to date in the shared Marvel Cinematic Universe (MCU).

However, if time is of the essence, or you just want the bare essentials, here are the movies you should have seen, watch in preparation, or at least be familiar with before you see Infinity War:

• Captain America, Iron Man, Incredible Hulk, Thor (for character origins; as listed = chronological order; Hulk could probably be skipped if you know even a little about Bruce Banner/Hulk)
• Marvel's The Avengers (2012)
• Avengers: Age of Ultron
• Guardians of the Galaxy (Vol. 2 is mentioned briefly, but not terribly relevant)
• Captain America: Civil War
• Doctor Strange
• Black Panther (the story of this movie is not really relevant, but the setting and characters introduced are front & center in the climax of Infinity War)
• Thor: Ragnarok (Infinity War essentially picks up right where it left off)

That comes out to about a full day worth of Marvel films (21 hours, more or less), but the investment is well worth it in the end.