As I mentioned a few weeks ago, I headed to Oregon recently for my sister's wedding. The wedding was in Corvalis so I was very lucky that I got an afternoon of exploration in Portland before my official MOH duties started. I visited Powell's Books in the Pearl District the last time I visited, and I could not wait to get back there.
Size-wise, it compares to Strand in NYC or John King in Detroit, but what I especially love about it is how wonderfully organized and well-laid out it is. I enjoy an enormous bookstore as much as the next literary hoarder; it can be really pleasurable to get lost in a sea of shelves and discover something you didn't even know you wanted. But on the other hand, book stores are a rarity these days, and for them to serve any practical function in an urban environment, folks need to be able to find what they're looking for. Powell's gives you the best of both worlds; it's a several story building, with different genres on each floor, so it's not hard to find at least the vicinity of what you're seeking, and it's just as easy to wander as you please.
I'm also impressed by how not-seriously they take themselves. That sounds like serious shade so let me clarify: so many bookstores put themselves up on pedestals because books are an expensive luxury (unfortunately) and reading is for serious, smart people (or something like that). Powell's seems to remember that reading is meant to be pleasurable, as is shopping for books, and they have no shame in stocking their shelves with toys, games, stationary, and other nifty book-adjacent merchandise. They have a nice cafe upstairs, a huge selection of magazines, and they devote fairly equal real estate to each genre. If you were a hardcore fan of sci-fi or fantasy, for example, this is the bookstore for you.