My favorites frequently change (and I don't think this is in order), but:

Catch 22 - Heller
The Good Earth - Buck
Atonement - McEwan
Where the Red Fern Grows - Rawls
LotR - Tolkien
Red Mars - Robinson
A Heartbreaking Work of Staggering Genius - Eggers
Hocus Pocus - Vonnegut
One Flew Over the Cuckoo's Nest - Kesey
1984 - Orwell

And at this point I don't have one particular favorite book, but I could read mythology/history (esp Greek) all day long.

There are a few non-fic that would make the list except that it's hard to claim something that's seriously depressing as a favorite.