Without question American Football is the prime sport in the United States in the second decade of the 21 st. Century.
The amazing thing about baseball, however, is that it is still tremendously popular despite each team playing almost 200 games in about the same number of days. There are 162 regular season games plus another handful of exhibition and preseason games. The fact that teams like Boston and New York and some others consistently draw over 30,000 to each game, is astounding.
Football is played once a week and stadiums pack in 50,000 to 100,000 fans. During that one week, a baseball team may play 7 or even 8 games and draw over 300,000 paying customers.
When viewed in this context, you could argue that baseball is still the number one American Sport. With that said, I am going to 'flip flop; and ask: what is the number one must see sporting event? A no brainer - "The Super Bowl".