Thanks for posting this thread, I can see in the other comments lots of books that I now want to read.

For me I don't have a one favorite book, but my take would be:

- 1984 from Orwell

- the Foundation series, from Isaac Asimov (and basically, in my taste any book from Asimov has some good chances to be good)

- Hyperion, from Dan Simmons (a good scifi writer as well).