For the third straight year, Delta SkyMiles is the best domestic frequent flyer program, WalletHub says. The personal-finance website ranked the 10 largest U.S. airlines based on 23 key metrics, ranging from the value of a point per mile to blackout dates for rewards flights.

Other findings include the following.

  • The average airline rewards program gives frequent flyers 12 percent off their airline spending. And they’re all free to join.
  • Hawaiian Airlines offers the most rewards value to frequent flyers ($19.95 per $100 spent), with Alaska Airlines coming in a close second ($19.87 per $100 spent).
  • Four of the 10 largest airlines are offering more rewards in 2018 than in 2017, sweetening the pot by an average of 20 percent. They are Delta Air Lines, Southwest Airlines, United Airlines and Sun Country Airlines. American Airlines’ rewards are virtually unchanged.
  • Five of the 10 largest airlines—Alaska Airlines, JetBlue Airways, Spirit Airlines, Hawaiian Airlines and Frontier Airlines—are offering fewer rewards in 2018.
  • Delta Air Lines and JetBlue Airways are the only two major airlines whose miles do not expire because of inactivity.
  • Seven out of 10 airlines do not impose blackout dates on tickets purchased with miles.
  • Six out of 10 airlines allow rewards-program members to earn and redeem miles with partner carriers. United Airlines and Delta Air Lines have the most partners.

U.S. airlines are expected to carry more than 600 million passengers on domestic flights this year.