Has to be hockey. If you want to make the NHL around 25, you'd have to start playing around 8 or 9. You have to completely exert yourself on the ice for around 45-60 seconds if you're a forward, 60-75 if you're a D-man, you need to master skatework and be able to check, hit, speed up, shoot the puck with near perfect balance, then go another shift a few minutes later. It takes so much concentration and so much energy just to play the game, let alone get good at it. Even the goaltenders need amazing vision, reflexes, concentration and confidence. Most players in the NHL really only continue to endure the game because they want the crown jewel of hockey, all 35 pounds of the Stanley Cup. It's a giant fucking mug, people. It has to be the most badass professional sports reward out there.
Oh, need I mention fighting? Fighting on ice is surprisingly even more taxing than fighting on rubber. Most players who fight actually need more than the 5 penalty minutes for fighting that they're given just to recover from the fight. Hell, if your fight lasts over 2 minutes, you're done for the period. The coaches intervene more often than the refs.