I do not like ice cream. Having said that, Tillamook would be the brand of first choice. Umpqua would be second. Both good Oregon brands.
You'll never know the best unless you've sampled Costco vanilla. Downside is it's 2 gallons. 13 Cult-Favorite Costco Foods That Deserve The Massive Hype Soooo good .
Since there is no real Washington-centric brand (apart from some very localized dairies far too small to serve as state-wide mascots -- Molly Moon, Cougar Gold, Edaleen, etc.), Tillamook could get top billing here. I'm now noticing significant amounts of Umpqua east of the OR border. Though some secessionists are trying to pick up that border and move it to their west.
Trying to eat healthy and watching calories etc my favorite is Krogers Orange Sherbet Ice Crème. Used to be a Ben and Jerry's Cherry Garcia fan but the sugar, cholesterol, fat and calorie content put it off limits.