Great athletic performances are usually measured against the event when the stakes are higher and pressure greater; the Olympics, the World Series, the Superbowl, etc. and usually they far surpass the nearest competition.

Olympics:(some previously mentioned)
Jesse Owens - track
Bob Beamon - broad jump (loved his reaction)
Wilma Rudolph - track
Eric Heiden - skating
Michael Johnson - track
Olga Korbut - gymnastics
Mark Spitz - swimming
1980 Hockey Team

Baseball:
Reggie Jackson's 3 homeruns in the World Series
Don Larsen's perfect game in the World Series

Golf:
Tiger Woods dominance in the Master's and US Open
Jack Nicklaus' number of career victories and Major victories.
Byron Nelson's consecutive tournament wins.

But some atheletic performances are great because of the courage and determination that the individual showed. Like those people that finish the Ironman/woman Triatholon in total exhaustion.