I would say the Marvel characters have equal importance, for the most part, but Peter is, to me, the most relatable, and certainly crucial to the franchise. He is a teenager, and like all teenagers today, he has to deal with bullying, romantic troubles, balancing homework and his hobbies (which just happen to include web-slinging around Queens and saving old Dominican ladies), and he's smart. As in, nerdy level smart. To be a nerd today is common, and certainly not an insult, and yet we are still getting practically no recognition outside of the kid who has every Star Wars movie memorized and plays D & D with his two friends after school. Peter has given us that relatable feel that we, as a nerd nation, lack. However, he's not simply a 'nerd'. Because no human is just one thing, he is also sweet, enthusiastic, and a loyal friend/nephew. He has been through hard times, and, best of all, he didn't start out as the perfect, all-American Friendly Neighborhood Spider-Man. He started out as a teenager who had just gotten mutant powers. He didn't always use his new abilities the way he should have, and he even had to live through his uncle's death. He has matured with time, and simply wants what is best for everyone. Many psychological professionals have discussed the idea of relatable traits in a hero, and I feel that it is a key factor in Peter's importance. As our beloved Stan Lee said, "he could be anyone under that mask". It's not just Peter who's relatable; it's every person who has ever donned the identity of Spider-Man. There is something about the timelessness of Spider-Man that appeals to people, as well as a sense of youthful innocence.
You can look at Spider-Man from literally any angle, and he is still the most relatable Marvel character. My personal favorite movie representation of him is the MCU's, with Tom Holland. (Clearly, the MCU shared things with every other version, but I'm referring to just their Spider-Man. There will be parallels, as it is the same character.) Obviously, the MCU is not play-by-play of the comics, but I feel that they transferred Peter into modern-day Queens seamlessly, and worked him into the lives of the Avengers incredibly well. Who can't relate to being reprimanded by a mentor for being over-eager and messing something up? Who doesn't keep secrets from their guardian? Who doesn't have a huge crush on someone, and feel nervous talking to them? We all have a friend like Ned, who we can act nerdy and strange around. Even the pop culture references that so annoyed Tony make him that much more relatable. The reason why Spider-Man is important is the same reason why everyone tries to create him, and why everyone was heartbroken in Infinity War (but who wouldn't be? "I don't feel so good, Mr. Stark", and "I don't wanna go!" are killers. Also, to 'peter out' is to slowly fade into nothingness, so there's that). He's relatable as Andrew Garfield, Toby Maguire, or any other actor. He's relatable in every single comic. He lets us feel great about being us, and if you ask me, that's about the most important thing a hero can do.