Answer:
Without a doubt, cycling.
Cycling means you get your butt on a saddle for six hours, keeping in mind breakaways, sprinting, and bike handling. Hills and long miles make this sport extremely hard and challenging. Though millions believe that all you do is pedal, that is a HUGE understatement. These people know nothing about the challenges about this internationally acclaimed sport.
Cycling is THE most physically demanding sport--NOT basketball, baseball, football, or any other American appreciated sport.