One Movie you MUST watch – Avengers: Infinity War

The prequel to Avengers: Endgame works as a stand-alone film and can be understood without seeing any other Marvel movies. However, Endgame will be much harder to follow without having seen Infinity War.
Honorable mentions

Thor: Ragnarok
Avengers: Civil War.

Aliens and Space

Guardians of the Galaxy Vol. 1 and Captain Marvel

Villain Thanos and the different alien species he subjugates only exist in a few Marvel movies, and several Earth-centric plots don’t even acknowledge life from other planets. Guardians of the Galaxy provides the best all-in-one package of important information, while Captain Marvel chronicles some of Earth’s limited contact with Aliens.
Honorable Mention

The 1st Avengers Movie (2012)

Endgame Fan-Theories

Ant-Man and The Wasp
Doctor Strange

The Marvel Cinematic Universe provides great conditions for fans to try and use context from other films to support theories of future plot points. Two films feature aspects used in some of the more popular and plausible Endgame theories: the “Quantum Realm” featured in Ant-Man and the Wasp or the “Dark Dimension” featured in Doctor Strange. Ant-Man also gets bonus point for the meme “Thanus Theory” (which is worth looking up for a laugh).

Producer Chad’s Avengers: Endgame Cheat Sheet