In terms of numbers watching - the athletics is by far the largest - however this is due to the size of the stadium and also several events are on the go at any one time in the athletics disciplines. So while athletics as a general discipline is the most popular, the single most popular event is harder to officially recognise. Swimming is generally sold out at every Olympics, as is the gymnastics (however this was not the case in Athens). Given the size of the stadium and the global media coverage the Mens 100m is however the generally regarded to be the most watched event at any Olympics.
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for summer Olympics http://www.mahalo.com/olympic-sports-list
and for winter Olympics
http://www.topendsports.com/events/winter/sports/
I would say beach volleyball. It is a very intense sport that more people should play. Gymnastics pulls in a larger crowd.