The Skyclub at SEA is insanely nice. I'd take it over the hot mess Centurion any day of the week, especially with the new access restrictions at SEA that make it effectively useless.

I'd say the nicest lounge in the system is by far the JFK lounge, which is apparently the model for all new Alaska Lounges going forward. They had some really nice cocktails, the barista station is legit (and better coffee than anywhere else in the building) and the atmosphere is great with nice views and lots of natural light. Hell I'd say in terms of atmosphere it's nicer than the Concorde Room. Even the bathrooms are well done with fancy toiletries instead of the "picked this up at Walgreens" I see at PDX and SEA.

The only downside is the free food seems to be slim pickings and maybe even slightly less than the other lounges, but not sure if it was opening day anomalies or not. They do have premium options for purchase at a reasonable price, which considering the state of the rest of the terminal (terrible) is fine.