I think these types of lists will always be subjective based on where you place your importance. Kim Gallagher was far and away the best traditional H.S. athlete. God only knows what she could have run in a 2-mile if she ever ran it, or a mile or even an 800 against pros or the even the best H.S. athletes. Mary Decker, who in my mind is slightly overrated, she ran a lot of fantastic times, but she had a very abbreviated H.S. career, yet her indoor 800 and her win against the Russians is as big as anything a H.S. athlete can accomplish and you have to be mindful of the passage of time. The best young 800m runners will line up in a couple of weeks on a much better track than Decker ran on and I doubt they will run faster than 2:01. If Mary Cain never ran another step, she is in the top-3 and she has another indoor and outdoor season remaining. Cain has run incredible times, but it is hard to not factor in the equally incredible opportunities she has had, albeit she has made those opportunities available and made the most of them.
The top-3, Cain, Gallagher and Decker should be a closed issue simply because they are the only H.S. athletes whose dominance was such that they could not be challenged in their specialty against H.S. athletes and were literally running world class times in their teens. You simply can't say that to an extent about Hasay, Fairchild, Plumber, etc., the XC phenoms and the athletes who ran incredible times in events that H.S. athletes don’t routinely contest and if you were struggling to beat H.S. athletes in your own state. For some perspective, just looking at Cain, Gallagher and Decker ‘s 800m time alone, the the H.S. 2-mile, 5K, 10K and marathon records don’t compare.