American football, without a doubt. The Super Bowl is always the most watched program in the country and the college football championship gets huge ratings. For basketball, it honestly depends on region. I'm from a state that's huge on college basketball but could care less about the NBA, but it's different for others. Baseball's popularity comes more from attending the games than watching them on TV, except for the World Series.

The pecking order is usually American Football, Basketball, Baseball, Hockey, then Soccer.