I went with the NCAA tourney, because that's probably the greatest few weeks in sports. Just about all of the games are unreal, and it's great what CBS has done in the past few years giving most cable companies a few extra channels so that fans can watch any of the games that they want.
The Super Bowl is a close second, just because of the size of the spectacle that it has become.
The NHL playoffs as a whole are also up there, just because the heat gets turned up about 73 notches to the point where the ice almost melts. That's about the only time when I routinely watch hockey.
The Masters is my fourth favorite event, just because there always seems to be some sort of great storyline.
That being said, an MMA 'Super Bowl' would probably trump all of these. If we could get every top org in the world to come together to give us a ton of superfights, then I'd probably sit down for an entire day and watch that.