That's a rather complicated question. (Gridiron) football gets the most television viewers, but baseball gets the most ticket sales (but then again, it has a longer season). From my own view those two sports generally top basketball, but between them I don't think you can make much of a distinction (but I think baseball is often romantically viewed as America's sport). If you look beyond professional leagues to college level (football and basketball specifically-- not really baseball), then it adds another layer of complications. A lot of big college teams have cult-like followings from alumna passing it down through their families--it's really intense.

Long comment is long but I think it's an interesting question :)