The undisputable leader in efficiency among airlines is Southwest Airlines ( LUV ). It has the highest gross margin of 27.2% and Net Margin of 17.1%. The second in the list in terms of the Margin is Alaska Air Group ( ALK ). But ALK is much more expensive if measured by Price-to-Free-Cash-Flow ratio. I would add ALK to my Watch List in case its price drop making the price of that company attractive. Despite  lower Gross Margin and Net Margin both, Delta ( DAL ) and United ( UAL ) are favorably priced with Free-Cash-Flow yield over 5%.  In conclusion, I add to my Watch List fours stocks: LUV, DAL, UAL and ALK