The biggest sporting event in the world is undoubtedly the Futbol World Cup. Other notable events are perhaps the Summer Game Olympics and the American Football Super Bowl. In the land down under (Australia), the biggest sporting event would probably be the Rugby World Cup.
I have to admit that Mike Syr Sutton has a point with March Madness and the USA Men's College Basketball Championships. It is indeed my favorite event each year. Still, I can admit that World Cup Futbol (Soccer) is more important to most of the world outside the USA.