Fixed. That's a better term, these characters go beyond movies and kids, some of these characters have had far larger exposure to far more people in a much longer time. Spider-man has enjoyed that for decades now, same goes with the hulk. Iron Man has only truly been popular for the past 5 years, and arguably that's mostly for a certain generation. I'd likely get more response with Spider-man or hulk with someone over 50 or someone from another country (for example, spider-man is HUGE in asian territories) it's to do with all sorts of things, tv shows, comics, merch, history etc.
Not saying Tony Stark isn't in the top 3-4, but he's certainly not spider-man level, not by a long shot.