Captain America: The Winter Soldier
Directed by Anthony Russo, Joe Russo. With Chris Evans, Samuel L. Jackson, Scarlett Johansson, Robert Redford. As Steve Rogers struggles to embrace his role in the modern world, he teams up with a fellow Avenger and S.H.I.E.L.D agent, Black Widow, to battle a new threat from history: an assassin known as the Winter Soldier.
Arguably the least comic bookish of the MCU movies, I believe this was the movie that gave Marvel the impression that the Russo brothers could manage characters in a believable and enjoyable way.