In NW Washington, it's CHKG Vancouver, BC. But as you head south to Everett, KXXO Olympia starts booming in. And KXXO comes in almost rock solid from Everett to the Oregon state line.
KXXO is going to relocate their tower to better reach the Seattle area. When that happens, it's likely their south end coverage is going to suffer. But I also think a little unmentioned part of it has to do with making room for Portland to have another full power station on 96.3.