I'd go with wrestling as the most physically demanding, in general. You can have some variations of other sports (Tour de France, ultra-marathons, etc) that can slowly wear a competitor down, but wrestling is the most intensely physically demanding sport.
When I ran cross-country and distances, the toughest distance was always the half-mile. It was too short to really pace yourself and too long for a sprint - it was just 2 minutes of hell. Wrestling takes that a couple steps further - after 2 minutes of hell, you get a 30 second break before your next 2 minutes of hell, and then your next, and then maybe an overtime period just in case you forgot you were in hell.
It also just feels so much more degrading when you lose a wrestling match than a race. You lose a race to someone ranked among the best high school runners in the nation's history you don't really feel that bad (there were two kids in our local area chasing Steve Prefontaine's national high school record in the two-mile). In wrestling, losing to a three-time state champ whose never been beaten still sucks.
Edit: I'd also second Physic's Whiz's vote for indoor soccer being the most fun.