To me, these two athletes were the most impressive at the Beijing Olympics:
Michael Phelps
Usain Bolt
His coach said Usain could have run the 100m in 9.52 seconds instead of 9.69 seconds if he had only run through the finish line as he did in the 200m.
One gymnast that comes to mind as comparing (slightly) with Michael and Usain is two-time World Champion Dmitry Bilozerchev.
When he won World’s in 1983 at age-17, there had never been a prodigy like him. It seemed impossible at the time.
Gymnastics Coaching.com 