Everywhere you go these days, there are superheroes. In movies, on t-shirts, around theme park rides. They're and fill a sort of vacuum in American culture for mythology. It's very tempting take all sorts of stuff from American pop-culture and medievalize it, but I think superheroes would have a distinct advantage. First is that after a collapse of civilization, comic books would be the most accessible. Television and motion pictures would simply not be consumed as much, as no device to play them. People would still be able to use books, in theory. However, after a generation or so, the literacy rates would drop. Comics, at least those that would physically withstand the stresses of the time, are low-tech but visual enough. While most folklore takes decades, even centuries to codify, having reached millions of people at one point kind of evens that out.
I rated comic book superheroes by four factors: Recognition, (How broad an audience did they reach?) Seniority, (How long have they been around?) Iconography, (How easy would a character be recognized using the most basic of artwork?), and of course, Non-Modern Elements. The conceit I'm using is that the more a character uses elements that are explicitly tied to modern elements, then more you have to kind of "plug in", and the more of a chance that character has to have stuff "plugged in". So someone with a lot of technology and operating by a modern civic code might not translate as well. I've basically just referenced characters with live action movies and TV Shows. For the record, I haven't included Thor. He would obviously have a place in the new Medieval world--certainly in Germanic Europe and likely in America as well. (And many Marvel characters probably get a boost by being connected with him)