I know this has been talked about a lot, so I'll try and keep it brief.
One of the biggest problems in the past for Marvel Studios has been the constant changing of composers and the lack of leitmotif between films. I think all the examples of inter-film use are...
1. Alan Silvestri's Captain America theme briefly plays in the very first moments of Captain America: The Winter Soldier. Other than that, however, Henry Jackman introduces his own Captain America theme.
2. Alan Silvestri's Avengers themes play briefly in Avengers: Age of Ultron just after the opening action sequence, when the quinjet returns to New York. Other than that, the main theme by Danny Elfman is supposed to apparently be a version of the Silvestri theme, but I fail to really hear it.
3/4. Brian Tyler, who composed about half of Age of Ultron, reuses both his Iron Man (when the Hulkbuster armor debuts) and Thor (as Thor saves falling cars during the battle of Sokovia) themes from Iron Man 3 and Thor: The Dark World.
5. Henry Jackman reuses his Winter Soldier theme in Civil War, but his Captain America theme curiously never shows up. (instead there is a sort of "Team Cap" theme that pops up)
6. Guardians of the Galaxy Vol. 2 became the first Marvel sequel to bring back the OG composer, reusing the main themes properly.
7. Mark Mothersbaugh makes great use of multiple pieces from previous films in Thor: Ragnarok. Brian Tyler's music from Thor: The Dark World, which played during Frigga's funeral (and may have played during Loki's "death" scene, I can't remember) is played diegetically by the chorus during the theater reenactment of Loki's "death." More of Brian Tyler's music, from the scene in Avengers: Age of Ultron, when Natasha gives a "lullaby" to Hulk, is replayed when Thor tries to do the same in the arena. Then, Patrick Doyle's original Thor music plays at the end of the film, as Thor takes the throne of Asgard.
The credits list Hulk's Lonely Man theme as well, though I just rewatched the film and didn't notice it anywhere.
8. Silvestri triumphantly returned, along with his Avengers themes, for Infinity War, and he also uses Ludwig Goransson's Black Panther music for the introduction of Wakanda.
9. Ant-Man became the second franchise to immediately bring back the OG composer for the second film, Christophe Beck, complete with his themes.
10. Silvestri returned again for Endgame, this time using his Captain America themes multiple times (Tony debuting the new shield, Cap wielding the hammer, and Steve's last scene), which I don't believe showed up in Infinity War. Also, the Tesseract theme from the opening moments of the first Avengers also shows up in the track "The How Works," as the team discusses their plans to retrieve the stones.
He also used Beck's Ant-Man theme, Pinar Toprak's Captain Marvel theme, and I believe even some of Mark Mothersbaugh's music representing Asgard from Thor: Ragnarok, though the use of the former two is not on the soundtrack!
11. Michael Giacchino is set to return to Spider-Man in Far from Home, only the third franchsie to immediately reuse the OG composer.
Did I miss anything? What would you like to see return in the future?