Rugby, they never stop and wear no pads.
I don't do this sport but i think gymnastics would be the hardest, because you need a lot of upper-body to do bars. And if you do a back flip wrong on a balancing bar and fall it would hurt really bad. You need to stay focused and you have to do everything perfect so you don't hurt yourself
Easily baseball you need incredible hand eye coordination and if Michael Jordan couldn't even be a decent player in the minor league you know it's hard.
Baseball. You have to aim, (and catch, and throw HARD) hit hard, (which requires a lot of skill) run fast, slide well, and not pass out from dehydration. Personal experience, not pleasant, but rewarding.
It takes far more skill and finesse to run and control a ball with your feet than it does to run with a ball in your hands.
Polo? First buy a horse...
