Douglas Adams Hitchhiker's Guide is definitely one of my all-time favorites.

One of my more modern favorites is Neal Stephenson's Anathem. It's very dense, and my first time through it took me 100 pages just to be able to start reading at a normal pace with all the made-up vocabulary, but it's an absolutely fascinating novel featuring awesome stuff like quantum physics and parallel universes.

I was much more of a fantasy genre reader growing up, so I'm still sorely lacking in a lot of science fiction knowledge. It's something I've been meaning to work on for years now.