I'll qualify my vote for "bowling" by stating that I don't consider the last 2 options as real "sports." By definition, a sport involves physical exertion, so that rules out cards, scrabble, and (gasp) the spelling bee (albeit NOT the hot dog eating contest). I'd call those "games" or something.
Auto racing is really in a league of its own, due to the use of machines (come on you guys with numbers tatooed on the back of your pickup windows, I know it's hard to drive that many miles in a car with no AC -- it's just DIFFERENT than the other options).
Also, you might qualify this as "least favorite sport TO WATCH or TO PLAY." My guess is that there would be a lot of difference -- e.g. my vote. Another example, anyone who's ever played Soccer knows that it's constant action. The problem with TV-viewing is that most of the action is passing or ball control and doesn't usually result in a goal, which is pretty much the sole reason most people watch sports. (Who's favorite part of Sportcenter isn't "Top Plays?")