Quote Originally Posted by Kyono
If we're talking team sports, I'd say Cricket. Games that last 5 days in a row, where they have to concentrate mentally every second of the game, as well as a helluva lot of physical exertion, especially for fast bowlers. Couple this with the fact that Domestic competition has games that last 4 days as well, International series can run up to 5 matches long, some back to back, and this isn't even including the ODI's and OD competitions in domestic seasons. Plus the fact that any time Shoaib spears down a ball at 100mph that then rises up at your throat and could potentially kill you, yeah. Cricket.
I'd say cricket too. It's one hell of a challenging sport for both batsmen and bowlers, there's so much precision required for bowling...the tactics are insane. And batting? One days might not be too bad, but when you have to stay there for five days, with some batters at the crease for more than a day (depending on how well they play ) it'd be dead tiring.

Edit: Although I feel sorry for footballers, because they have to head the ball...it must be excruciating for the bald ones.