Ten years ago it'd have been Spider-Man, but the MCU has completely altered the way Marvel markets it's products. Iron Man is in, and a part of, every piece of merchandise you can think of nowadays. Spidey has more cultural relevance and nostalgia attached to him being the face of Marvel, but if it's a decision of putting Iron Man or Spiderman's image on something, Tony is going to get the nod these days.
Same reason I'd argue that Batman is the true face of DC, despite what DC actually says - they put more marketing behind Batman (more shows, more games, more movies, more products, more comics) than Superman.
Unless you're talking about in-universe relevance, in which case Superman is clearly the winner and Marvel has the Avengers as no single character really encompasses that position or merits that focus.
In every major multiversal storyline, it's always Superman that gets the job done.
We've seen him become DC's ultimate superhero (yes, arguably greater than Logoz/Mages-empowered Spectre.) He has the greatest story ever told amongst any superhero in the entire comicdom, bar none.
About merchandise, that's a different topic altogether. Batman captures the intrigue of both comic and non-comic fans due to him being a mere human with no supernatural enhancements. I see him as REALISTICALLY the ideal superhero in the eyes of most people. But Superman IS the ideal comic book superhero.
It's the same with Spider-Man. He arguably has the best story amongst all other Marvel superheroes. I can see the reason why he's been viewed this way. But the thing is, he's never been established as the character that finishes off the big bad in multiversal events. None of Marvel's superheroes have, imo. That's the problem.
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"Farewell, Damos... Ash, Pikachu... And you. All of my beloved." -- Arceus
Pete or Tony, as others have said. In a symbolic sense, Steve is their center. But in terms of widespread appeal, the other two.
That's a problem? I consider it a bigger problem that a company would have someone whose job it is to finish off every big threat. Superman is a great character when treated like the mythic symbol as well as the man. But overuse in-universe is a real issue with both companies and their biggest draws.
I mean, for all the attention its gotten, Secret Wars is a literal rehash of a story that's already been told, complete with the same big bads and heroes (give or take a couple on either side). Both are guilty of this idiocy.
Like before his movie he wasn't an unknown or anything be he was definitely one of those characters that the public only somewhat knew about and was under characters like Hulk or Captain america. Now he's being debated against Spidey in terms of being the face.
I mean in terms of merchandise and stuff Spider-man sells miles and miles ahead of literally any other superhero (It was like 1.3 billion dollars in 2013, whereas the second highest was Batman at like $400 million or something), and that's without being front and centre in the MCU films which have caused the Avengers to become a huge thing.
In universe is obviously different, with Cap being the moral centre of the universe.