Spider-Man's been Marvel's main mascot since about the mid-60's. WWII era, it was Cap, since most of the Marvel characters people give a **** about weren't invented yet. You could argue Wolverine and recently Iron Man are secondary mascots. But Spider-Man's popularity is the longest standing, and he generally establishes and exemplifies Marvel's "flawed heroes with human problems" philosophy. Great Power and Great Responsibility and all that.
DC's mascot is between Superman and Batman, a little muddier than it is with Spider-Man and Marvel. I think you could genuinely make a case for either of them.
That said, Iron Man probably got the biggest glow up from B-Lister to their 2nd or 3rd most known hero currently.