Any talk of gulf oysters is proving Island Bro's point about how good they are from his 'hood. That said, limiting one's self to the Pacific Northwest is ignoring the likes of Baja California, Humboldt, New Brunswick, and Nova Scotia.
And in general the smaller the better. I'm usually of the take that the more salinity the better. What makes them so special is that they can evoke so much of the sea. Just a couple drops of lemon and you're good to go. Those big fuckers are meant to be grilled or broiled with stuff that gives them flavor. Sure, a good oyster bingo can be tasty, but it can never be magical.