Really depends on what type of activities you enjoy. The answers already posted are great!
I tend to love the fall and winter myself - there's so much to see with autumn harvest and grape crush for the huge wine industry east of the Cascade Mountains. (Washington State is 2nd in the nation for wine production and first in organic produce!)
I'm a snowboarder so thought I'd mention some of the new happenings: Crystal Mountain has a new Gondola this year to whisk you to the Summit in comfort. White Pass Ski Resort has doubled their land use (and it wasn't easy!) to create two new quad lifts and Snoqualmie Pass Ski Resort has added two new lifts to backcountry which was not accessible in the past.
Spring brings the blossoms in the orchards, as well as the incredible Skagit Valley Tulip Festival and summer - well there is nothing more beautiful than a sunny Washington State day!
Visit all seasons - we have something for everyone!