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.