Some slight changes from the original release order put crossover scenes/events closer to their eventual pay-off, without confusing the in-universe chronology any more than it already is.
Asterisks mark movies pulled forward from their original release order:
- Iron Man
- Iron Man 2 *
- The Incredible Hulk
- Captain America: The First Avenger
- The Avengers
- Thor: The Dark World *
- Guardians of the Galaxy *
- Iron Man 3
- Captain America: The Winter Soldier
- Avengers: Age of Ultron
- Captain America: Civil War
- Spider-Man: Homecoming *
- Black Panther *
- Doctor Strange
- Guardians of the Galaxy Vol. 2
- Thor: Ragnarok
- Avengers: Infinity War
- Captain Marvel *
- Ant-Man and the Wasp
- Avengers: Endgame
Iron Man 2
As noted elsewhere, one of the last scenes in Iron Man 2 includes news footage from the university incident in The Incredible Hulk, suggesting that the climax of Hulk happens after Iron Man 2. Tony's pre-credit appearance in Hulk works just as well if we assume he's visiting General Ross in his capacity as a consultant on the Avengers Initiative.
In addition, Iron Man 2 opens with Tony's press conference at the end of Iron Man playing live on TV in Ivan Vanko's apartment, then skips forward six months, so it flows on quite nicely after Iron Man.
Thor 2 and Guardians 1
Thor: The Dark World begins (Dark Elves flashback excepted) with Loki being brought back to Asgard as a prisoner, allowing us to see the immediate fallout of The Avengers.
Its mid-credits scene features the Collector, so it's nice to follow it with Guardians of the Galaxy to get immediate pay-off there, and to keep the space-based movies together.
Nothing in Thor 2 or Guardians spoils anything in Iron Man 3 or Winter Soldier, which are both set on earth, and have some thematic parallels.
Spider-Man and Black Panther
Spider-Man: Homecoming and Black Panther can slot in immediately after Civil War, so that we get to know the new characters — Spider-Man, in particular, follows on directly from that character's involvement in the airport fight, and Black Panther’s post-credit sequence pays off Civil War’s mid-credits sequence.
Nothing in either movie particularly affects the rest overall in-universe chronology, and this order also:
- puts the Doctor Strange mid-credits sequence closer to its payoff in Thor: Ragnarok, and the movie as a whole closer to the appearances by some of its characters in Infinity War and Endgame;
- leaves the tonally-similar space-based Guardians 2 and Ragnarok right next to each other; and
- means the Ragnarok mid-credits sequence is followed immediately (well, after a short Goldblum comedy interlude) by its payoff in Avengers: Infinity War.
After the post-credits scene of Infinity War, we can meet Carol straight away, then spend time with Ant-Man right before his fairly large role in Endgame.