thinking too much into this...
first of all, the most marketable person is probably going to be American. doesn't matter how good of an athlete as long as they're at the elite, "WC qualifier" level.
gotta be good looking
gotta have been through the high school and collegiate tiers on a high level (sorry Ajee, sorry Drew)
gotta either be white or a very attractive minority.
can't be from the multis or field events: sprint or distance only
can't be too shy on camera
at this point there aren't a ton of people still in the running. jordan hasay has got to be up there, obviously her main plus is the fame from high school xc/track. you could have a guy like Trayvon Bromell but in the US if you're a sprinter you're expected to be winning olympic medals and he didn't.
nick symmonds was pretty marketable but his event the 800m is too diverse for him to stand out based on his race. if you had a white dude competing in the sprints against mostly africans/blacks, he'd be a huge star.
so even after this, I don't know. as a distance runner from the west coast I'm biased towards Hasay.