In reality, the most athletically challenging sport is martial arts. I am not talking about getting a black belt. In almost all cases, a black belt means nothing, and is not even a remote judge or testament to skill level. This is proven by the police reports that show the average black belt to be only 1% more likely to win in a real fight. I'm talking about broken bones, torn muscles, and many other types of injury. No matter what style you take, you still have to go through grueling training to actually master the art of combat, not some class with nice padded floors where you are completely safe and the opponents are submissive and not actually trying to hurt you.
I would have to say wrestling. When wrestling and in other forms of fighting sports you use every muscle of your body in a match. The conditioning for the sport includes hours of extreme work to the point of body failure, throwing up is expected, and there are no cuts, only quiters. During a match you are faced up with someone your own weight and their goal is to make you hurt and your goal is to do the same. Its an intense sport where 6 minutes on the mat may feel like the longest time of your life and when its over, even the sound of food is enough to make you sick. Not to mention that most of the time you cant even eat during season in order to maintain your weight class. This include skipping thanksgiving dinner in order to make the winter tournaments. Spitting in cups to lose water weight and running with 2 sweatpants on 2 sweatshirts a jacket and trashbag over your body to lose weight before a match. This is just the surface of wrestling. It is in MY books atleast the toughest sport.