The Complete Hitchhikers guide to the Galaxy. It's one book that contains the four books of the original trilogy (it only lacks Mostly Harmless, which was mostly crap, anyway).
For second place it would be the bunnies of Watership Down.
High Fidelity by Nick Hornby. Holy crap, it's like Hornby had been watching my life through hidden cameras, writing down everything he saw. Only he made all the purely depressing parts kinda funny.
SF/Fantasy-wise? Probably Slaughterhouse Five by Kurt Vonnegut.
I'm going to cheat and pick my favourite four (in no particular order):
LotR - 'nuff said
Magician - the first in the Riftwar Trilogy by Raymond E. Feist. Brilliant fantasy adaptation of Eastern and Western cultures.
The Way of Wyrd - Brian Bates. Wonderful blend of myth, history, faith and friendship.
1984 - George Orwell. A genius critique of authoritarianism. This book was banned in the former Soviet Union and only available to members of the Politbureau (the Soviet Union's ruling body).
I'm getting too old for favorites but my favorite book always used to be Dune so I'm sticking with it. It's an amazing story that simply expands my horizons every time I read it.
Mind you, for me the story ends with Dune. I'm trying very hard to forget the sequels. As for the spin offs, I would never bother.
There's lot of books I love to read, so I couldn't pick out a favorite. There are certain books I'm proud to have on my shelf: The Lord of the Rings, Band of Brothers, Shogun, and others, but I don't think there's one that stands head and shoulders above the rest as my "favorite."
That's my favourite Narnia book. "Out of the Silent Planet" is my favourite CS Lewis book.
(Incidentally, the Silver Chair shares a line with a short story by Damon Knight made famous by an episode of the Twighlight Zone. Both were published around the same time. Does anyone know who came up with the line first? I don't want to say the line, as it's a spoiler, but if you don't know what I'm talking about, you won't know the answer anyway.)
All time favourite: Good Omens or Hitchiker's Guide to the Galaxy
Close behind it would be "Ender's Game" by Orson Scott Card.
Like Tarrasque Wrangler said, it's like the author was watching my life, but he made all the sucky parts incredibly dramatic and interesting.
For single favorite book I'd have to say When HARLIE was one 2.0 by David Gerrold. I've reread that book so many times that the spine is falling off. For favorite author and series however, I'd say that Stephen Brust and his Dragaera series is the epitome of modern fantasy.