For science fiction (not including fantasy). All of these in my opinion are masterfully written, fascinating works. I picked out ones that I think are best of the best and that also span a wide range of themes and styles.
Title: Dune
Author: Frank Herbert
Year Published: 1965
Aspects: Factions, conquest, political maneuvering, action, mysticism
Title: Starship Troopers
Author: Robert Heinlein
Year Published: 1959
Aspects: Excellent miliary SF by a SF luminary. (Also, for a timely AH connection, Heinlein served on the USS Lexington, which will be appearing in the upcoming Coral Sea scenario.)
Title: Hitchhiker's Guide to the Galaxy
Author: Douglas Adams
Year Published: 1979
Aspects: Hilarity
Title: Neuromancer
Author: William Gibson
Year Published: 1984
Aspects: Started the cyberpunk genre.
Title: 1984
Author: George Orwell
Year published: 1949
Aspects: What an overbearing, socialist government will get you.
Title: Valis (or Ubik)
Author: Philip K. Dick
Year published: 1980 -- Valis (1969 -- Ubik)
Aspects: All of Dick's stuff is heavily psychological and . . . well . . . unique. (Sort of how in fantasy there is only one H. P. Lovecraft.)
Title: Snow Crash
Author: Neal Stephenson
Year Published: 1992
Aspects: A very interesting blend of cyberpunk, real world action and conspiracy; and both humorous and serious.
Title: The Stars My Destination
Author: Alfred Bester
Year Published: 1956
Apsects: What makes this great is Bester's style and craft of writing, not really the plot or anything else.