True story, I find Spider-Man to be the most relatable superhero within both Marvel and DC. Not only do you get to read about him fighting the likes of the Green Goblin and the Juggernaut, but you also see him juggle it with an everyday teenage life: bills, jobs, women, parental issues, schoolwork, etc. And from time to time it'll all get to be too much for him. A villain will win or he'll lose someone he really cares about, and he'll think about giving up the mask. But at the end of the day he always remembers that "With great power comes great responsibility" (Easily one of the most quotable lines from a comic book ever) And he'll stand back up and go back out there, fighting the good fight. Out of everyone, Iron Man, Captain America, Thor, the X-Men, Spider-Man is literally your average person, perhaps a tad more selfless and self-blaming, if he/she gained superpowers. Beyond that, any amount of happiness that Peter Parker gains in his life is through hard work and dedication. Beyond his superpowers, which arguably cause him more harm than good, nothing is handed to him. He has to work his *** off for it and work his *** off to keep it, even being forced to deal with a public that hates and fears him the majority of the time. He's an excellent example of doing something not because it's easy or will earn you a reward, but because its the right thing to do, and having to live with the fact that you may very well never earn the credit you deserve for it. Because again, unlike Iron Man and Captain America, in his civilian life he's just an average joe and in the mask crowds aren't going out of their way to praise him and give him awards.
As bad as the Ultimate Line of Comics were from Marvel, one of the best scenes (Not just for that line, but for all comic books) was this one
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You've got everyone else, the Ultimates/Avengers, the X-Men, the Fantastic 4, etc. worrying about Magneto and trying to find and take him down, and completely focusing on that to the point that the citizens who are suffering from the massive tidal wave Magneto unleashed and dying by the hundreds. While all of them went off to take down the villain, Spidey stayed behind and single handedly worked himself to death to save whoever he could, person by person. And this is again the one guy who realistically speaking isn't going to get any thanks for what he does. I really think that panel more than any other shows why he's a great hero. Besides being a wisecracking smartass, he's just a good person who understands the common man, because he is the common man. Most often, when you've got Galactus or some big cosmic bad guy threatening to destroy the universe, Spider-Man is the one person who will say something about just how insane all this is (Which we as readers understand, because a dude that eats planets is some other type of ****) but he'll then still patch himself up, get a new costume on, and rush out there right alongside everyone else, despite the fact that he's vocally made it clear that he thinks this entire situation is crazy.
I could probably go on and one, but I find his comics to be the perfect combination of adventure/fantasy and realism, to a level that other characters just aren't able to bring to the table.