Here's my nomination for Greatest Athletic Performance of the 20th Century.
Now, I'm not a big sports fan, and unlike Robin I certainly don't know very much about horses, but Secretariat utterly destroying the competition by 31 lengths at the 1973 Belmont Stakes is probably the closest I'll ever come to witnessing perfection. It's one of the defining moments not just in horse racing, but in sports. On that day, Secretariat would have beaten any race horse in the history of the sport.
Secretariat still holds the world record for a mile and a half, after 33 years. In the 1973 Kentucky Derby, he did something no horse before or since has ever accomplished--he ran the race with negative splits, completing each quarter mile faster than the one before it.
Horse guy or not, that video never fails to put me in a state of awe.