I have to go with Soccer players, those guys are in tremoundous shape, running all the time, barely stopping and always going at full speed. Track and Field stars are also great shape atheletes, they train all year to stay in great shape.
I would have to say that ironmanners are the most fit athletes in the world. These guys train hours and hours a day and can outdistance pretty much any other athletes from any other sport, not that it's all about distance, but they are just all around fit. As far as the most athletic athletes, I would have to say professional track and field sprinters take that crown.