Now that that's out of our system...allow me to introduce you to some of the best, so you don't have to bother with all the rest.
Hatchard's: I could live in here. It has a classic, even old-fashioned, vibe but is chock full of fresh new books. The modern reprints of vintage Agatha Christie editions pretty much epitomize this effect for me. Their poetry and history sections, the ones I gravitate towards, are also very good. It's no surprise they're the Royal purveyors.
London Review Bookshop: Lovely, light, and welcoming, with a tearoom to boot. I judge a lot of shops on their sparse poetry shelves, but the LRB doesn't disappoint. They're conveniently near the BM as well.
Waterstones: I was surprised by how much I liked this major chain; it feels much nicer and cozier than U.S. brands. They have an extensive selection and lavish attention on their layouts and displays, which are dotted with recommendations and quirky categories. A very fun spot to browse. (Still, I probably won't buy much from them because who wants to pay full price?!)
Stanford's: Somewhere in the annoying retail trail of Covent Garden lies this oasis of maps and books. And three stories of them, no less! They have everything you could imagine in the way of inventive travel guides, globes, literature, and even gadgets. I highly recommend that you ignore all the fancy brands on Long Acre and just pop in here to browse. Excellent for planning a trip, or for hopping around on the world-map floor like a small child.
Persephone Books: Like the goddess Persephone rising from Hades in the spring, this shop (re)prints neglected fiction and non-fiction by mostly female writers of the twentieth century. Their bold and lovely design aesthetic sees each book issued with a grey jacket covering brightly patterned endpapers and a matching bookmark. I love the idea, the execution, the cozy shop space with creaking wooden floorboards...everything.
I'll be sure to add to this list if I visit any others that catch my fancy. Here's the thing: while I love bookshops, I also have limited disposable income. So as much as I love browsing, I generally rely on libraries to borrow and Amazon/secondhand stores to buy. Am I killing the very thing I love? Probably. But...eh. A girl's gotta do what a girl's gotta do.