Who's the greatest athlete that ever lived? That's easy. Secretariat.
After 29 years, his records at the Kentucky Derby, Preakness and Belmont Stakes still stand (as do some other track records he set). His never seen before and never seen since 31 length (~250 ft! ?) margin of victory at the grueling Belmont Stakes will never be touched.
Later, computer analysis revealed that his physical anatomy, geometry, etc. was as perfect as a horse's can possibly be. Further analysis revealed that his gait/stride was also as perfect and efficient as a horse's can possibly be. Couple that with the fact that after his death, his heart was found to be approximately TWICE the size of a normal race horse's and you've got the perfect/ultimate equine running machine.
But all of those things put together are not all that's needed to make an athlete, especially a distance runner, truly great. What are also needed (and arguably even more important) in such an athlete are the desire and the indomitable will to push his or herself to outcompete and win and you see those things in spades in this video: Watch how, in every race, Turcotte never puts the whip to him but Big Red never stops trying to go faster and pull further away from the pack. Even at the Belmont, when he was so far ahead he literally could've trotted the last furlong and still won, he kept pouring it on and kept pulling further and further ahead of the pack. But Turcotte wasn't the one making him do it; it was his will and desire. It was Secretariat pushing himself to get to the finish line faster. In that sense, his heart was WAY more than twice the size of other horses. To my mind, he was, without contest, the greatest athlete that ever lived.
Watch this video and be in awe...