NPR has release the "Best" Sci-Fi/Fantasy novels ever.  Of the list 
(which you can access here), I have read 26 of them and am in the middle of reading the 27th with 
The Song of Ice and Fire.  I find it strange and confusing that sometimes they choose entire series as one book like 
Lord of the Rings while others they just include one book like 
Dune even though there is an entire series of 
Dune books.
I can't quite figure out their criteria, but for those that want to take a look please take a look at the list, or just look below for a list with no descriptions:
For those curious, books in 
BOLD I have seen.  Books with an (m) after them I have seen the movie, so have a pretty basic idea of the premise
1. The Lord Of The Rings Trilogy, by J.R.R. Tolkien
                     2. The Hitchhiker's Guide To The Galaxy, by Douglas Adams
                     3. Ender's Game, by Orson Scott Card
                     4. The Dune Chronicles, by Frank Herbert
                     5. A Song Of Ice And Fire Series, by George R. R. Martin - (reading...)
                     6. 1984, by George Orwell
                     7. Fahrenheit 451, by Ray Bradbury
                     8. The Foundation Trilogy, by Isaac Asimov
                     9. Brave New World, by Aldous Huxley
                     10. American Gods, by Neil Gaiman
                     11. The Princess Bride, by William Goldman
                     12. The Wheel Of Time Series, by Robert Jordan
                     13. Animal Farm, by George Orwell
                     14. Neuromancer, by William Gibson
                     15. Watchmen, by Alan Moore
                     16. I, Robot, by Isaac Asimov - (m)
                     17. Stranger In A Strange Land, by Robert Heinlein
                     18. The Kingkiller Chronicles, by Patrick Rothfuss
                     19. Slaughterhouse-Five, by Kurt Vonnegut
                     20. Frankenstein, by Mary Shelley (m)
                     21. Do Androids Dream Of Electric Sheep?, by Philip K. Dick
                     22. The Handmaid's Tale, by Margaret Atwood
                     23. The Dark  Tower Series, by Stephen King
                     24. 2001: A Space Odyssey, by Arthur C. Clarke - (m)
                     25. The Stand, by Stephen King
                     26. Snow Crash, by Neal Stephenson
                     27. The Martian Chronicles, by Ray Bradbury
                     28. Cat's Cradle, by Kurt Vonnegut
                     29. The Sandman Series, by Neil Gaiman
                     30. A Clockwork Orange, by Anthony Burgess
                     31. Starship Troopers, by Robert Heinlein - (m)
                     32. Watership Down, by Richard Adams - (m)
                     33. Dragonflight, by Anne McCaffrey
                     34. The Moon Is A Harsh Mistress, by Robert Heinlein
                     35. A Canticle For Leibowitz, by Walter M. Miller
                     36. The Time Machine, by H.G. Wells - (m)
                     37. 20,000 Leagues Under The Sea, by Jules Verne  - (m)
                     38. Flowers For Algernon, by Daniel Keys
                     39. The War Of The Worlds, by H.G. Wells
                     40. The Chronicles Of Amber, by Roger Zelazny
                     41. The Belgariad, by David Eddings
                     42. The Mists Of Avalon, by Marion Zimmer Bradley
                     43. The Mistborn Series, by Brandon Sanderson
                     44. Ringworld, by Larry Niven
                     45. The Left Hand Of Darkness, by Ursula K. LeGuin
                     46. The Silmarillion, by J.R.R. Tolkien
                     47. The Once And Future King, by T.H. White
                     48. Neverwhere, by Neil Gaiman
                     49. Childhood's End, by Arthur C. Clarke
                     50. Contact, by Carl Sagan -(m)
                     51. The Hyperion Cantos, by Dan Simmons
                     52. Stardust, by Neil Gaiman 0 (m)
                     53. Cryptonomicon, by Neal Stephenson
                     54. World War Z, by Max Brooks
                     55. The Last Unicorn, by Peter S. Beagle -(m)
                     56. The Forever War, by Joe Haldeman
                     57. Small Gods, by Terry Pratchett
                     58. The Chronicles Of Thomas Covenant, The Unbeliever, by Stephen R. Donaldson
                     59. The Vorkosigan Saga, by Lois McMaster Bujold
                     60. Going Postal, by Terry Pratchett
                     61. The Mote In God's Eye, by Larry Niven & Jerry Pournelle
                     62. The Sword Of Truth, by Terry Goodkind
                     63. The Road, by Cormac McCarthy
                     64. Jonathan Strange & Mr Norrell, by Susanna Clarke
                     65. I Am Legend, by Richard Matheson - (m)
                     66. The Riftwar Saga, by Raymond E. Feist
                     67. The Shannara Trilogy, by Terry Brooks
                     68. The Conan The Barbarian Series, by R.E. Howard
                     69. The Farseer Trilogy, by Robin Hobb
                     70. The Time Traveler's Wife, by Audrey Niffenegger
                     71. The Way Of Kings, by Brandon Sanderson
                     72. A Journey To The Center Of The Earth, by Jules Verne -(m)
                     73. The Legend Of Drizzt Series, by R.A. Salvatore
                     74. Old Man's War, by John Scalzi
                     75. The Diamond Age, by Neil Stephenson
                     76. Rendezvous With Rama, by Arthur C. Clarke
                     77. The Kushiel's Legacy Series, by Jacqueline Carey
                     78. The Dispossessed, by Ursula K. LeGuin
                     79. Something Wicked This Way Comes, by Ray Bradbury
                     80. Wicked, by Gregory Maguire
                     81. The Malazan Book Of The Fallen Series, by Steven Erikson
                     82. The Eyre Affair, by Jasper Fforde
                     83. The Culture Series, by Iain M. Banks
                     84. The Crystal  Cave, by Mary Stewart
                     85. Anathem, by Neal Stephenson
                     86. The Codex Alera Series, by Jim Butcher
                     87. The Book Of The New Sun, by Gene Wolfe
                     88. The Thrawn Trilogy, by Timothy Zahn
                     89. The Outlander Series, by Diana Gabaldan
                     90. The Elric Saga, by Michael Moorcock
                     91. The Illustrated Man, by Ray Bradbury
                     92. Sunshine, by Robin McKinley
                     93. A Fire Upon The Deep, by Vernor Vinge
                     94. The Caves Of Steel, by Isaac Asimov
                     95. The Mars Trilogy, by Kim Stanley Robinson
                     96. Lucifer's Hammer, by Larry Niven & Jerry Pournelle
                     97. Doomsday Book, by Connie Willis
                     98. Perdido   Street Station, by China Mieville
                     99. The Xanth Series, by Piers Anthony
                     100. The Space Trilogy, by C.S. Lewis