It's a tough one to call. Any sport that relies on cardio fitness and one muscle group has to be discounted. Running/football for example. Boxing has to be up there in terms of the number of muscles it uses, most of us could run for a few minutes, but try boxing whilst running on the spot and you're soon burnt out. Plus you need to get the balance of speed/strength/stamina. Tennis is incredible - 5 sets, 5hrs plus of high intensity running and hitting, which puts nearly every muscle under strain. That's got to be up there.