Captain America might be the first Avenger, and Iron Man might have been the first star of the Marvel Cinematic Universe, but the Hulk is the most crucial and pivotal piece of the original Avengers line-up. The Hulkhasn't exactly had a great time at the movies, sure, but he's what makes The Avengers feel bigger than they are. He turns them from a group of reasonably strong and skilled people to comic book heroes.
The Hulk is what makes the Avengers an event, and brings the all-important hype. Because when you think about it, it's not "we have a Hawkeye," "we have a Black Widow," nor "we have a Thor," it's "we have a Hulk."
Let's go back to the beginning, shall we? 2008's The Incredible Hulk made the impossible happen and brought Iron Man and the Hulk together. It was the first real crossover of what we now accept as the MCU. Though the Hulk may not be the anchor to the franchise that Iron Man is, it did not feel like a connected universe until Iron Man himself made a cameo appearance in a bigger character's film. It was one of the rare post-credits scenes that legitimately surprised as The Incredible Hulk had no signs that it was taking place in a larger universe. The idea of a united universe of films of this scope didn't even seem tangible until then.
Bringing in the Hulk would be for naught, however, if the Hulk did not contribute anything of substance in the films. Thankfully, the opposite is true. Not only does his inclusion make each film he's in feel bigger just by the character's very nature, the Hulk plays a critical role in the narrative. The Hulk adds a much needed element of controlled chaos that often sets the story in motion, and regularly shifts the balance back in the Avengers' favor.
In The Avengers, the Hulk is absolutely vital in fighting the upcoming threat. His arrival towards the end of the film actually sparks the climax as the Hulk was the only one who could feasibly take on the humongous Chitauri spaceships, because it's not like Hawkeye or Black Widow were going to take them down with arrows and bullets. Without the Hulk, it would have been a losing battle. As the "we have a Hulk" line basically posits, Hulk was the only one needed to rival that force.
Age of Ultron further proves that the Hulk is the best as when he's their enemy, the Avengers don't really stand a chance. Even with Iron Man using the Hulkbuster armor, a brainwashed Hulk far outclasses the original line-up. The character went on to serve an even more important function of threading an emotional through line that parallels our heroes with Scarlet Witch and Quicksilver. Remember his emotional breakdown as he reacted to both being used, and frustration at his lack of control? That's the same arc we eventually see Scarlet Witch explore toward the end of the film and in follow-up appearances
The Hulk was sufficiently built up as the best of the original line-up over the course of three or so films, positioning him as an unparalleled powerhouse, so his defeat in Infinity War helps cement just how strong of a threat Thanos was even without the Infinity Stones. Because if the best Avenger could not defeat Thanos, then none of them would be able to.
Those are just some of the many examples of the Hulk's importance (and what makes him stand above the others) before even factoring in just how vital to everything Mark Ruffalo's Bruce Banner is. Ruffalo adds a humanity and tragic levity to each film. His portrayal of the anxious Bruce constantly fighting back the Hulk's brutality adds a much-needed suspense to the Avengers line-up. This suspense makes every one of his appearances hold a sense of wonder and anticipation as Banner could transform into the Hulk at any moment. Banner is often added to dialogue-heavy scenes to be bounced off of, and the unpredictability of his other half makes him a point of interest beyond even what's being said.
Characters interacting with Banner often show a different side of themselves, and this is especially true in Stark's talks with him in Age of Ultron or with Thor in Thor: Ragnarok. It's all emphasized even more so thanks to Ruffalo's natural charisma and charm. Ruffalo makes Bruce Banner such an electric, affable person that it makes the Hulk stand out over the others in the line-up even more.
Also, you may not even realize it, but the Hulk was already in your subconscious way before any of the other Avengers were. Before the character made its appearance in The Avengers, the Hulk already had two films under its belt -- something the other MCU heroes had to cram in within four or so years. Before that, even, there was a popular television program, which led to The Incredible Hulk becoming a pop culture icon thanks in part to the character's superpower being easy to remember and reference.
The fusion of merit and importance inside and outside of the Marvel Cinematic Universe makes Hulk the best Avenger. The Hulk may not be one of the "trinity," but the Avengers would not be as big of a deal as they are now without him. I mean, "incredible" is his middle name.
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