I prefer auto racing over any other regular sport.  Out of all forms of auto racing, I find NASCAR to be the most interesting with F1 as a close second.  With that said, I don't follow either much anymore as I just don't have the time to watch it.  I don't have much time to watch any sports these days.  I went to my first Cardinal game in over a year last night, not because I don't want to go anymore, but I just don't have the free evenings to do it.
As far as NASCAR being the largest spectator sport, I don't know if it still applies today, but I can say for sure that back at it's peak (around 2002 to 2007), it was definitely the biggest spectator sport in the country.  Back then I didn't have anything to do on my Sundays, so I would watch most of the races.  And I've been to enough races to tell you that it's no more of a redneck experience then going to any profession football game (especially if you include the tailgate experience).