Walmart was not as trashy as peopleofwalmart.com led me to believe, although the staff are clearly paid less, the customers were of a lower socio-economic demographic bracket, and for the first time in my life, as I exited a supermarket someone checked my receipt and went trough my cart. Big store, especially by UK standards, but not a mind-blowing mini-city like I had heard. They're apparently based outside cities, which is true for Seattle.
Costco WAS the mini city that blew my mind though. Wow. Definitely my favorite, if I had a car.
Safeway and Quality Food Center are mid-range (and close to my place.)
Trader Joe's is smaller, does a lot of its own brands, specializes in organic food, aiming at slightly above mid-market.
Target is not the same company as in Australia. Their mix of food, apparel and home goods confuses me.
Whole Foods and Metropolitan Market are very expensive, but lots of really nice foods.
Pike Place Market is tourist attraction is expensive, but again, lots of really nice and exotic food.
Local farmers' markets on certain days of the week. Common, popular, vary a lot. Ones closer to the city tend to be more like artisan markets with little range in fresh produce and high costs.
Have a specific food question? Ask away.