1. Football/Soccer by a long shot.
2. Boxing (Boxing doesn't have a specific league or singular organization, so its scattered. But everyone around the world is ready for a good scrap).
3. Tennis. No crazy fan following like football or cricket, but everyone tunes in for Grandslams and Federer/Nadal/Sampras/Agassi/Serena. Tennis has produced huge stars.
4. Cricket, only for the huge numbers it brings in from India, Pakistan and Bangladesh. It is also the second biggest sport in Australia and England, I believe. It is largely ignored in other European and American countries.
5. I'd say Rugby/Golf.
UFC will replace Boxing though. Fighting is the only sport everyone in the world will tune into watch like it or not. Its the most basic form of competition. Boxing used to the king pin, but the rules make it limited and now people are moving onto better things.