As others have said, WN typically has the lowest unrestricted fares. Restricted round-trip fares tend to be similar from airline to airline, but Southwest may offer a lower one-way fare.

Southwest has the best schedule frequency, with about 14 flights every weekday. AA and UA have fewer flights, but still a reasonable schedule frequency.

If you are uncertain about your travel plans, Southwest is the way to go. If you are forced to change your ticket, you only pay the fare increase (if any). They do not tack on a $100 ticket change penalty like American does.

In terms of service, American wins hands down. The seats are assigned, the boarding process is civil and even the ex-Reno planes have MRTC, meaning a 33-35 inch pitch. The 737-800 is the plane to take if it fits into your schedule.