I agree with the Stephenson and Gibson, but to try to add more unique titles:

Tad Williams - Otheland (series)
Nick Sagan - Idlewild
Neal Stephenson - Cryptonomicon
Douglas Adams - The Hitchhiker's Guide to the Galaxy
Asimov - I, Robot

Idlewild was pretty good by itself, but I've never managed to get into the sequels. The Otherland series is excellent. You can't go wrong with Stephenson (I haven't read the Baroque Cycle or Zodiac, but I think I've read everything else of his and it's all gold). THGTTG is funny, and that's why you read it, not for the story. Asimov is great. Period.

Stranger in a Strange Land gets an honorable mention just because I want people to grok where I'm coming from.