For free access, any. Ive gone to "The Club" United Club, and DL Skyclub. The Club and United Club has similar amenities like cereal, muffin, and beverages. The one major advantage at United Club is that their agents are quite knowledgeable with UA system and are helpful if my flight had been on United. Obviously, the United club is busier when UA flights are departing, while The Club often busier when international flights are departing. The Club A Concourse lounge is at the end of A Concourse and can be difficult to get to for some flights, while The Club at S concourse (the old Northwest WorldClub), is easily accessible for flights in S concourse.
For ultimate experience, DL SkyClub at the A concourse is by far, the best lounge. The A Concourse Skyclub is one of Delta's Flagship lounge, with 6 shower suite, full bar and kitchen. During mornings, kitchen is serving up Oatmeal, Cereal, fresh cut fruits, yogurt, Hot entrees (norms: Bacon, eggs, potatoes), breads (fresh baked scones, croissants) and breakfast beverages including starbucks espresso via starbucks automated machines. In the afternoon, there are often five different mixed and create your own salad, soup, sandwiches, and pasta or potato hot dish. Bartender can make wide variety of complimentary beverages, as well as premium collection of higher priced drinks. SkyClub at the S concourse do have two shower suites, but lacking a full kitchen, have less food offerings. If you have a lot of time, I'd encourage you to splurge $59/person visit at DL Skyclub.
You can see the full review of the DL SkyClub here.
Review: Delta SkyClub Seattle Airport - One Mile at a Time
Jiburi