Definitely Hassan or Gidey. Perhaps Kipyegon by track fans.
Of the US, the only one that comes close is Molly S.
There really aren't many female distance runners with a global appeal, as you have to be known among the marathoning world, or breaking records, for joe-public to know anything about you.
Maybe Courtney Dauwalter. What female distance runner has more Instagram followers than she has (597K)?
Sidney McLaughlin is at 1.1M, but she isn't a distance runner.
Sifan Hassan (273K), Faith Kipyegon (183K), and Elle St. Pierre (90K) are stars, but smashed in popularity by someone like free runner Hazal Nehir (469K).
Parker Valby all the way. An “it girl” is defined as a young woman who is attractive, popular, and has a great personality. No distance runner fits that description better than Valby.
Francie was an amazingly tough dedicated runner, but what I loved about her was her big heart: she never forgot people who supported her or touched her life. She once came up to me at the trial in Eugene and greeted me warmly, although I had only been a newly assistant coach who I not monitored some workouts she did with our top boys, at a high school where, although a student there, she trained and competed with a private club. Gracious and ferocious on the track.