The answer is easy. If you are a top level athlete then boxing is the hardest. When training up to 12 weeks for a fight boxers run 5-7 miles per day six days a week. After running they go to the gym Spar, hit the bags and do other weights and gym work for two hours a day at least.
None sense- think of the training that goes into rider the Tour de France. It takes years to build the endurance just to complete such an event. Add to that the anaerobic fitness and need to climb steep long mountain passes that even cars can struggle up. 12x 3 minute rounds of boxing are tough but nothing like the demands of a Grand Tour
Cheerleading was the toughest for me I guess, takes hours to train physical aspect, build up rapport with your team, memorize the dance moves etc. The training I did for that was actually tougher then army training, rock climbing and track & field.