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.