I couldn't resist posting this,  the "100 Best SF/Fantasy Books" based on a poll of NPR listeners. The meme that's going about its to bold the ones you've read.
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 - and everything else he's ever written.
4. 
The Dune Chronicles, by Frank Herbert 
5. A Song Of Ice And Fire Series, by George R. R. Martin - only the first 2.5 before becoming horribly depressed.
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 - only the first seven or eight before becoming horribly bored.
13. Animal Farm, by George Orwell 
14. Neuromancer, by William Gibson 
15. Watchmen, by Alan Moore 
16. I, Robot, by Isaac Asimov 
17. Stranger In A Strange Land, by Robert Heinlein - and everything else he wrote.
18. The Kingkiller Chronicles, by Patrick Rothfuss - Just the first one, so far
19. Slaughterhouse-Five, by Kurt Vonnegut 
20. Frankenstein, by Mary Shelley 
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 
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 
32. Watership Down, by Richard Adams 
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 
37. 20,000 Leagues Under The Sea, by Jules Verne
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 
51. The Hyperion Cantos, by Dan Simmons 
52. Stardust, by Neil Gaiman 
53. Cryptonomicon, by Neal Stephenson 
54. World War Z, by Max Brooks 
55. The Last Unicorn, by Peter S. Beagle 
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 - sad to say, yes
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 
66. The Riftwar Saga, by Raymond E. Feist 
67. The Shannara Trilogy, by Terry Brooks - only the first book, I found it hopelessly derivative
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 
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 - this is where the library began
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