Collage football is probably the most watched followed by NFL, Baseball, Basketball
In regards to watching a baseball game live at the stadium - i've been told by an american that unless you are stoned or drunk (or both) it is like watching pain dry. And this was an american speaking. I've watched a few minutes of MLB games on foxtel and I would believe it - watching a five day cricket test match is more exciting.
Watching NFL is no problem for me. no offense to the American's here, but NFL (Gridiron) is just a dumbed-down, soft version of Rugby League. and if you can follow the League, you can definitely follow NFL.