The best book I've ever read is perhaps "cheating" because it's "The Next Whole Earth Catalog" and, as the name says, it's a catalog of (mostly) books.
For fiction, came here to say the same about Gene Wolfe, the whole solar cycle series by him might honestly be the (for now at least) absolute pinnacle of both the sci-fi and fantasy genres. Great world design, great characters, great writing (and variety of voice, the way he pulls off different characters writing is just brilliant, it really feels like a series written by several completely different authors.) The kind of books you read and reread and taken something new out of them every time. I'd honestly give just about anything to be able to go back into them completely blind again.
(You can see it for yourself in all it's glory here: https://www.wholeearth.info/p/the-next-whole-earth-catalog-f... )
Other than that I'd have to say the Tao Te Ching.
(The best fiction book I've read is almost certainly "The Book of the New Sun" by Gene Wolfe. It's in the league of Tolkien and Dune.)