A little while back, Gizmodo did a ranking of all the Avengers characters, more from a comic book perspective than a cinematic one. They put Captain America in the top spot, saying that he’s come to epitomize the Avengers and has been their leader for most of their existence. That’s true enough, particularly when we limit the discussion to the bulk of the comic history. Given that a whole new generation of superhero fans is getting to know these characters primarily through the movies, it’s fair to ask if Captain America the real head of the modern Avengers? And if he’s not, who is? Let’s break down some of the candidates.
Iron Man would be the main competition for Captain America if we’re looking purely at the MCU. And there’s a pretty good argument to be made. He kicked off the entire MCU with 2008’s Iron Man, and has appeared in more movies than the rest of them. He’s also unquestionably the leading presence on screen thanks to Robert Downey Jr.’s endless reserves of charisma. Furthermore, we’re seemingly about to see him taking on a sort of mentorship role in Spider-Man: Homecoming as he instructs young Peter Parker (Tom Holland), which will make him seem even more like a team leader. The only arguments against Iron Man are that he’s not the labeled “Captain” in comic lore, and that at even in the movies we sometimes see Tony Stark defer to Captain America’s leadership.By the end of Civil War it seemed as if they were on a pretty even playing field, leading different factions of the Avengers but working together.
Cap is the de facto leader of the Avengers and has been for a long time. However, in the movies Captain America doesn't always leap out as the kind of presence that would look like the head of the world's most powerful superteam. Chris Evans’s approach to the role lacks the infectious joy of Downy Jr.’s, and he often seems a more sulky and less fun than the members. That said, this can set him apart as not only responsible, but also as a leader, and other modern Marvel entertainment clearly places him at the center of things. When it comes to video games Captain America has generally been a more valuable and important character than Iron Man. One of the leading online titles in when it comes to online slots is based on The Avengers, and it not-so-subtly places the Captain at the center of its action, a space generally reserved for the leader. Themed slot reels and gaming apps can only go so far, but they do support the common idea that Captain America is the most important Avenger, even if he’s not always the most fun on screen.
Thor is kind of the pinch hitter of the group, for those of you who are baseball fans. He’s not always around, but every so often he’s called upon to come in and do some real damage. The thing is, he’s also quite literally a god, and descends to save Earth whenever there’s a real threat. You can make the case he didn’t appear in Civil War because Earth was not in direct peril. You could read the entire MCU as a journey of humans growing more powerful with weaponry and special abilities, and a god jumping into the fray now and then to make sure things don’t get out of hand. That’s probably gives Thor too much credit, but it’s one interpretation that would make him, in some ways, the “head” Avenger.
She’s not one that comes to mind for a lot of Avengers fans, but in the films there’s actually a strong case to be made—which is one reason why people have been begging for a Black Widow solo movie for years. One write-up made a list of how she compares to other Avengers, stating that she’s as daring as Iron Man, as selfless as Captain America, as complex as the Hulk, and as mighty as Thor. The takeaway was that she’s just as much of an Avenger as each of them. But might that very fact not make her more of an Avenger than any of them? Most of the Avengers don’t share core traits—it’s what makes them interesting. She’s something of a Swiss Army knife that incorporates the best of each of them. She also has closer ties to S.H.I.E.L.D. and the people who brought the Avengers together in the first place, and she’s arguably been more effective at forming teams than Captain America or even Nick Fury. She might be the best counter to claims that Captain America is the leader of this team.
These are the four that are really in contention. When you look at it, once you step outside the comics it's not at all clear that Captain America is necessarily the definitive leader.