Well, in my opinion, the astronaut. Look at the requirements:
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Pilot astronauts need a minimum of a bachelor’s degree in engineering, biological/physical sciences or mathematics. They also require a minimum of 1,000 hours pilot-in-command time in a jet aircraft.
Pilot astronauts need to be able to pass a NASA space physical, which includes the following items:
Distant visual acuity: 20/100 or better uncorrected, correctable to 20/20 each eye
Blood pressure: 140/90 measured in a sitting position
Height between 62 and 75 inches
For applicants applying to be Pilot astronauts, NASA strongly recommends flight test experience. While it is theoretically possible to obtain these credentials without going into the military, the majority of NASA Pilot astronauts come from either the Air Force or Navy.
The applicant must be free from any disease.
The applicant must be free from any dependency on drugs, alcohol or tobacco.
The applicant must have the normal range of motion and functionality in all joints.
The applicant must have visual acuity in both eyes of 100% (20/20) either uncorrected or corrected with lenses or contact lenses.
The applicant must be free from any psychiatric disorders.
The applicant must demonstrate cognitive, mental and personality capabilities to enable him/her to work efficiently in an intellectually and socially highly demanding environment
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Those are the minimum requirements, and as a result of a high number of applicants NASA can choose the cream of the crop.
Olympic athletes just have to, basically, be the best at exercising. Big deal.