Alaska Airlines Mileage Plan is fantastic because of its diverse partnerships. They are partners with many of the airlines both in the SkyTeam and oneworld alliances (and even some airlines like Emirates and Hainan Airlines that are in neither).
In the U.S. that means they’re partners with both American Airlines and with Delta, although with Delta they’re frenemies at best given the efforts Delta has made at building its own hub in Alaska’s home base of Seattle.