in the US for Marvel, Captain America, and maybe worldwide...he seems to have a good reputation. There was a time when Red Guardian may have been even more popular, as the Soviet version of Cap, and during the Cold War he may have been more popular.
The only mutant that might make the list is Dazzler, at least at the peak of her fame...popular not so much for being as superhero
Maybe Reed Richards (I don't think the other FF members are as popular).
Kevin Masterson during the Defalco run seemed to be a big fan of Thor...not sure how typical that was for kids though, so maybe he's a candidate.
If you factor in all the stories from mythology, Hercules might actually make the cut, and the list. In an old Avengers issue, he was talking to kids in central park who seemed to be big fans, except the one kid who was more of a Thor fan....
In DC its certainly Superman at the top, and you're right, Wonder Woman is probably #2. For a time, locally, Booster Gold was pretty popular, but that passed a LONG time ago.
Judging from DOS, Guy Gardner was popular with a certain segment of the population (teenage, rebellious boys who thought he was bad a#$), but I have a hard time thinking of anyone else high profile enough....Flash had a notoriously good reputation locally, but not sure how it was globally
Would say that Spiderman makes the list in Marvel, and that Superman is the top one in DC, as the general public and even the super villains know him as being one who plays fair and is the "Blue boy scout"