It's hard to pick a favorite book, but right now I'd have to say Hart's Hope by Orson Scott Card is one of my favorites. I found it to be unique, vivid, compelling, and memorable. I want to read it again, and I don't read very many books twice (there are too many that I've never read once!).
That's funny, I don't like Card's fantasy nearly as much as his sci-fi. But my sister loves his fantasy much better. We've already promised to not recommend books to each other.
I can never say what my favorite book is because I have recent ones that were incredible, then the old comfort books I return to repeatedly. My current favorites include Night Watch by Terry Pratchett and The Doomsday Book and To Say Nothing of the Dog by Connie Willis. And Three Men in a Boat, which I finally read after my brother-in-law loaned me his copy.