The icon?

Is Peter the face of Marvel or is Captain America?

Well, this is a Spider-Man board, so maybe this would be better suited for the general boards

but he's their most popular character, and I think he represents that "flawed/relatable" hero quality that Marvel came to prominence with then Captain America, who's often seems like he's a God among men.

Gotta be Peter surely? lol. In all seriousness though I do think it's Peter more than Cap because he's more of an every day man and he's more popular as well.

Everyone here knows Spidey, Batman and Superman, but only like 50% have even heard of Captain Murica.

Spidey and it's not even close. Seriously, it's been Spidey since roughly 1971 and nothing has changed that, not even the X-Men explosion of the '90's or Avengers more recently.