Delta SkyMiles scored the top spot among loyalty rewards programs of the largest U.S. airlines for the second year in a row, according to a recent study by personal finance website WalletHub.

Delta ranked ahead of second place Alaska Airlines, fifth-place United Airlines and sixth-place American Airlines.

The study analyzed frequent flyer programs of the 11 largest domestic airlines and scored them using 23 metrics, ranging from number of destinations served to value of reward miles or points. Airline loyalty programs were also compared on value earned for three different consumer profiles – frequent, average or light flyer – and Delta scored the highest in all three categories.

WalletHub praised Delta for being one of the only major airlines whose miles don’t expire due to inactivity, and for offering its rewards-program members the second largest number of partner carriers on which they can earn and redeem miles.