I would always recommend KEXP. They have a really varied set of shows and setlists, don't seem to be swayed by "we must play the top 40", genuinely care about music, and are super nice if you head into the building in Seattle (which has a public access space, coffee shop, bathrooms, and a SubPop store).

I've had three tours around the place just by telling the folks there that Im there from out of town. To be fair, I might have gotten the tours because I came all the way over from England the times, but it's my favourite place to hang out when I'm in Seattle.

Also Ken is the nicest guy ever, and he introduced me to Larry Mizzel Jr while he was broadcasting live.

Get the app, or use the player on their website, and hive then a try for an hour or do. The app has the ability to "heart" a track while it's playing live, which helps with discovery.