For the third year in a row, Alaska Airlines' Mileage Plan has been named the Best Airlines Reward Program in annual rankings of U.S.-based airlines by U.S. News & World Report.

U.S. News announced findings Wednesday of its 2017-18 survey that considers factors such as how easy it is to earn a free flight and what extra perks are awarded to members.

Delta's SkyMiles program moved into the No. 2 spot from fourth in last year's ranking.

Alaska retained the top ranking because its mileage-based earning structure makes it easier for travelers to earn miles at a faster pace than with revenue-based earning programs, according to the U.S. News announcement. U.S. News factors in airline quality rating scores in its rankings. Alaska's program is soon to merge with the Virgin America Elevate program, in wake of Alaska's acquisition of the other airline.

Delta's program moved up in ranking because of its competitive redemption rates for award flights and diversity of benefits, such as using miles for hotel stays, cabin upgrades, experiences and free flights.

Rounding out the top five rewards programs were JetBlue's TrueBlue program (No. 3); Southwest's Rapid Rewards (No. 4); and United's MileagePlus (No. 5).

Hotel rewards programs were also included in the rankings. The top rankings there went (in order) to Marriott, Wyndham, Choice, Hyatt and Best Western hotel chains. Marriott came out ahead for "the ease of earning points and its large network of hotels in popular destinations." Property diversity was weighted in judging of hotel rewards programs.