I maintain that college basketball is the greatest sport, but note something. Every season ends with an anticlimax: a meaningless post-tournament poll. Aside from a few curiosities (how high will they rank George Mason after making the 1996 Final Four?), it's of very little interest to anyone.
Compare to college football, where the final poll carries a bit of drama, and is still newsworthy in the BCS era. Those who advocate a playoff system -- and I am certainly among them -- might make college football look a little bit like college basketball, and that isn't all upside.
To answer your question...
Regular season (using the every-game-counts mentality):
1. NFL
2. College football
3. College basketball
4. NBA
5. MLB
Whenever the NFL institutes a Week 18, few will object.
Postseason:
1. College basketball
2. NFL
3. MLB
4. NBA
5. College football
Could be based on where I live, but I can't help but think there could be a more definitive ranking of these sports based on gambling action. Certainly helps that the gridiron is a once-a-week event, while the others are more frequent, harder to gauge, and less anticipated.