All of them.
But more seriously, give us some idea of what you are out here for/what area you plan on being in.
Seattle has about 60 breweries just inside off the city limits, and probably another 100 within easy driving distance, with each suburb having at least on brewery. You can get just about every single type of beer you want, though we are a little soft on German Lagers and Sours.
My favorite breweries in the area are:
Ruben's Brews - Ballard, Seattle
Black Raven - Redmond
Fremont Brewing - Fremont, Seattle
Schooner Exact - SODO, Seattle
If you really want a taste of the Seattle beer scene, I really recommend a beer bar, as you can get a wide variety of local beer without hitting each brewery on its own.
Favorite Beer Bars:
Pine Box - Capitol Hill, Seattle
Brouwers - Fremont, Seattle
Beveredge Place Pub - West Seattle
Favorite Bottle Shops - They also serve beer:
Beer Authority - Lake City, Seattle
Malt and Vine - Redmond
Special Brews - Lynnwood
I really think you can't go wrong with almost any brewery in the area. If you want to meet up for a beer, feel free to send me a PM.