I would say gymnastics. Gymnasts have to have extraordinary strength, power, speed, agility, flexibility, nerve, durability, and endurance to even attempt the sport let alone excel at it. There is a high risk of injury and even if they avoid a major injury, gymnasts must have a high tolerance for pain. On top of the physical aspect, gymnasts must have high levels of focus, drive, motivation, and intense desire to excel at the sport. It is by no means a sport that you can give less than 100% to ever and be good at.

You also have to consider how much time an athlete must put into the sport to make it to the highest levels. Most elite level gymnasts train around 40 hours a week. I can't speak for everyone but I don't know very many 15 year olds putting in hours like that for any other sports.