Depends on what type of books I'm looking for.
For fiction I would go to Chapters, to be sure. It's a big chain, lots of stock, and many locations. For Women's Studies, Sexual Health, or Social Issues I would go to either Venus Envy or After Stonewall. Plus Mother Tongue is good too but a further bike ride than the others.
There's a place in Toronto, owned by Chapters, but called the Worlds Biggest Book Store, and...I would believe it is. It's like walking into the warehouse for Chapters.
There's also a used book store in Montréal I discovered on my last trip there, it's right by the Bibliothèque et Archives nationales du Québec. I think the two might be affiliated. There was a surprising amount of German books, plus Spanish and Italian. We played it safe and stuck to the English.