This is a hard question to answer. You might say "Water Polo" or something obscure, but as soon as you could name an obscure sport someone else could invent a new one that would be completely unknown. "Least popular professional sport" might be something easier to qualify.
The most popular sport in America is Football.
The most popular sport in the US is football. Both NFL and College Football.
The most popular sport in the US is probably Football, but Baseball and Basketball are also very popular as well.
The NFL is the richest professional league in the US.