The Most Significant SF & Fantasy Books of the Last 50 Years, 1953-2002 according to Science Fiction Book Club.
gridlore brought this list to my attention. I've done things differently than he did. I've bolded books I've read and would recommend to others, italicized books I read but wouldn't read again and thus wouldn't recommend, and struck through any books I did not/could not finish and/or hated. There will be books I finished that are stricken through, but mainly because I want those hours of my life back.
1. The Lord of the Rings, J.R.R. Tolkien (Loved it, but admittedly slow-paced in places.)
2. The Foundation Trilogy, Isaac Asimov (However, I won't recommend anything past the original trilogy. Like many series, this should have stopped with the originals.)
3. Dune, Frank Herbert (Perhaps bucking the trend, this wasn't my favorite of the series. It runs a close second to God-Emperor. Wouldn't recommend anything past Chapterhouse, either. Do not attempt to write in an author's Verse after he or she is gone. It never works well.)
4. Stranger in a Strange Land, Robert A. Heinlein (Probably my favorite from this entire list.)
5. A Wizard of Earthsea, Ursula K. Le Guin (It's not bad, I just wouldn't probably read it again.)
6. Neuromancer, William Gibson (Think what you like about Gibson; cyberpunk started here.)
7. Childhood's End, Arthur C. Clarke
8. Do Androids Dream of Electric Sheep?, Philip K. Dick
9. The Mists of Avalon, Marion Zimmer Bradley
10. Fahrenheit 451, Ray Bradbury (It's important to read this, which is why it's assigned reading in high school literature classes. Still, once is enough.)
11. The Book of the New Sun, Gene Wolfe
12. A Canticle for Leibowitz, Walter M. Miller, Jr.
13. The Caves of Steel, Isaac Asimov
14. Children of the Atom, Wilmar Shiras
15. Cities in Flight, James Blish
16. The Colour of Magic, Terry Pratchett (I adore most of Pratchett's work. Some of it, I can do without. This is not one of those books.)
17. Dangerous Visions, edited by Harlan Ellison (Ellison can kindly get a stepstool and kiss my ass. I don't like his writing and I cannot stand him as a person. Foul little toad.)
18. Deathbird Stories, Harlan Ellison
19. The Demolished Man, Alfred Bester
20. Dhalgren, Samuel R. Delany
21. Dragonflight, Anne McCaffrey (I have never and likely will never see the appeal in the Pern series. I didn't like the founding concepts of the Verse and find McCaffrey's writing dull as dishwater. That said, she's still easier to read than Robert Jordan, whose books are useful only as firelighters.)
22. Ender's Game, Orson Scott Card (Genius of a work. Thanks to his personal politics, however, I can't recommend him as an author.)
23. The First Chronicles of Thomas Covenant the Unbeliever, Stephen R. Donaldson (Okay, I don't require a protagonist be likable. I can greatly enjoy an anti-hero. However, this one was about as deep as a layer of oil on a puddle. He's a two-dimensional piece of human excrement whose only emotion is spite. The last book of this I read ended with Covenant's death, and that's how I like to remember him. Dead.)
24. The Forever War, Joe Haldeman
25. Gateway, Frederik Pohl
26. Harry Potter and the Philosopher's Stone, J.K. Rowling
27. The Hitchhiker's Guide to the Galaxy, Douglas Adams
28. I Am Legend, Richard Matheson
29. Interview with the Vampire, Anne Rice (I read and enjoyed this, as well as the sequel. That said, I can't recommend it for fear of thereby unknowingly inflicting Rice's later works on someone.)
30. The Left Hand of Darkness, Ursula K. Le Guin
31. Little, Big, John Crowley
32. Lord of Light, Roger Zelazny (However, why this instead of the entirely of the Amber saga made the list, I do not know.)
33. The Man in the High Castle, Philip K. Dick
34. Mission of Gravity, Hal Clement
35. More Than Human, Theodore Sturgeon
36. The Rediscovery of Man, Cordwainer Smith
37. On the Beach, Nevil Shute
38. Rendezvous with Rama, Arthur C. Clarke (Not Clarke's best work, in my opinion.)
39. Ringworld, Larry Niven
40. Rogue Moon, Algis Budrys
41. The Silmarillion, J.R.R. Tolkien (Only recommended if someone read and loved the Lord of the Rings. Reading this first is the biggest mistake a person could make in attempting to embrace Tolkien.)
42. Slaughterhouse-5, Kurt Vonnegut
43. Snow Crash, Neal Stephenson
44. Stand on Zanzibar, John Brunner
45. The Stars My Destination, Alfred Bester
46. Starship Troopers, Robert A. Heinlein (There is very little Heinlein I wouldn't be happy to recommend, by the way. This one I definitely would recommend.)
47. Stormbringer, Michael Moorcock (This could have ended quite well with the earliest set. Moorcock diminished the character by continuing his adventures.)
48. The Sword of Shannara, Terry Brooks (I'm sorry. I know there are a lot of you who like Brooks, and this tends to be the top of the list to you. I just found it execrable. Truly, I want the time I spent reading this back.)
49. Timescape, Gregory Benford
50. To Your Scattered Bodies Go, Philip Jose Farmer