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Airline frequent flyer programs were among, if not, the first loyalty programs offered by U.S. companies. The original idea was to reward customers for booking flights with the airline. And the more you flew, the more reward miles you racked up toward a free flight.

The terms have changed considerably in the 30 or so years the programs have been around, and there have been many changes to them, in part because they became so popular and in part because airline margins slipped as fares fell.

Researchers at consumer website WalletHub ranked the frequent flyer programs at the 11 largest U.S. airlines on 23 different metrics, and the overall best program was found to be Delta Air Lines Co.’s (NYSE: DAL) SkyMiles program, which posted an average score of 59.9 out of a possible 100 across three usage scenarios: frequent flyer, average flyer and light flyer.

Alaska Airlines Inc. (NYSE: ALK) was ranked second with an average score of 53.01, and Hawaiian Holdings Inc.’s (NASDAQ: HA) Hawaiian Airlines was third with an average score of 49.23.

The two airlines that ranked at the bottom of the list were Spirit Airlines Inc. (NASDAQ: SAVE) with a score of 27.69 and privately held Frontier Airlines with a score of 31.83.

On a value basis, Hawaiian Airlines offers the most value, $21 for every $100 spent, and Alaska Airlines finished a close second with a value of $20 for every $100 spent.

Only two airlines offer reward miles that don’t expire: Delta and JetBlue Airways Inc. (NASDAQ: JBLU). Of the 11 airlines in the survey, eight do not impose blackout dates on tickets purchased with reward miles.

When asked whether customers or airlines benefit most from the rewards programs, one business school professor replied that she was “sure the airlines benefit more than do consumers.”

Some programs reward consumers based on miles traveled while others dole out rewards based on the cost of a ticket. Teasing out the details of these offers is time-consuming and fairly opaque. And that’s one more reason consumers remain loyal to one airline: the differences are often small and not easily determined, so why bother to shop around?

Fortunately, WalletHub has created a chart with its rating for each of the 11 frequent flyer programs and a score on each of the 23 metrics the researchers scored. The full methodology used to create the scores is also available there.

Credit card companies also partner with airlines to offer co-branded cards that reward consumers with points for air travel and other purchases, and often have big sign-on point bonuses attached. Last April we noted the best time to apply for this type of card.