if it's present day athletes, there's no contest that Christian Coleman and Justin Gatlin are at the top, ranked 1/2 in the world in track 100 m currently. That has to put them near the top of all time greats at Tennessee athletically.
(edit: for those that don't know, Justin Gatlin is incredibly 38 years old in this sport, putting him about 10 years older than the average competitor; #1 Coleman is 24). Even just competing at that age (think Peyton Manning) is incredible, but being the best of all the athletes at that age is almost unbelievable.
since this is in the football forum, you may not have meant to include a track star. I dont know who the greatest football athlete to ever play was, but i am looking forward to the debate here