Depends on what you like. The Dune and Foundation sagas are excellent (although a bit dated). For a more modern feel checkout Alastair Reynolds and Peter Hamilton.
I really enjoyed Ringworld by Larry Niven as well.And of course, the Hitchhiker's Guide trilogy in 5 parts :)
The first three Dune novels are particularly good. Dune has a tendency to get more philosophical in the later volumes (plans within plans within plans).
If I had to choose an all time favorite I would probably go with Kim Stanley Robinson's Mars trilogy (Red/Green/Blue Mars). An extremely well researched view of how Mars colonization and terraforming might proceed in a realistic way.
He also has a trilogy on the possible/likely consequences of runaway global warming (Forty Signs of Rain, Fifty Degrees Below, Sixty Days and Counting) but RGB Mars is definitely better.
Edit: "The content of Green Mars and the cover artwork for Red Mars are included on the Phoenix DVD, carried onboard Phoenix, a NASA lander that successfully touched down on Mars in May 2008. The First Interplanetary Library is intended to be a sort of time capsule for future Mars explorers and colonists." http://en.wikipedia.org/wiki/Mars_trilogy#On_Phoenix_spacecr...