On the football field, definitely Reggie White.
But the most extraordinary athletic feat I've witnessed live was when I was attending UT and went to a track meet in 1982, I believe. We were down on the track, and the 100 meter race featured Carl Lewis (Houston), Stanley Floyd (Houston), Willie Gault, and Herschel Walker. Lewis won going away, but the remarkable thing was seeing Walker finish in the middle of the pack amidst all of these high level sprinters. Watching a guy built like a linebacker move that fast is the most amazing thing I've ever seen in sports.