The hands-down winner would have to be Southwest, because four roundtrips of any length get you a free roundtrip anywhere WN flies. This would be the equivalent of 4,000 status miles on most other carriers (assuming every segment would get the customary 500-mile minimum). Many shorthaul routes are currently available for $71.50 rt, so an award ticket would cost you as little as $286.

Of course WN only flies within the lower 48, and then once you earn the free transcon you have to deal with the fact that you'll be stuck on a WN 737 for four hours.