Via Mixolydian Mode, a list of the Top 50 Science Fiction and Fantasy Books according to the Science Fiction Book Club. I will bold which ones I’ve read, and add a few comments.
1. The Lord of the Rings, J.R.R. Tolkien If you don’t like it, that’s a sad handicap for you.
2. The Foundation Trilogy, Isaac Asimov Justly famous; wouldn’t really work.
3. Dune, Frank Herbert Science fiction Islam with economics fun.
4. Stranger in a Strange Land, Robert A. Heinlein Fluff that thinks it’s deep.
5. A Wizard of Earthsea, Ursula K. Le Guin Actually deep.
6. Neuromancer, William Gibson Eh.
7. Childhood’s End, Arthur C. Clarke Not all that great.
8. Do Androids Dream of Electric Sheep?, Philip K. Dick
9. The Mists of Avalon, Marion Zimmer Bradley Pretty poor quality, and unrealistic too.
10. Fahrenheit 451, Ray Bradbury
11. The Book of the New Sun, Gene Wolfe Worth the pagecount, and good for vocab.
12. A Canticle for Leibowitz, Walter M. Miller, Jr.
13. The Caves of Steel, Isaac Asimov
14. Children of the Atom, Wilmar Shiras I think I’ve read this, but can’t remember.
15. Cities in Flight, James Blish
16. The Colour of Magic, Terry Pratchett The later books are much better, but the Luggage is awesome.
17. Dangerous Visions, edited by Harlan Ellison Influential, but pretty poor reading value.
18. Deathbird Stories, Harlan Ellison Well-written but depressing.
19. The Demolished Man, Alfred Bester Extremely clever and fun.
20. Dhalgren, Samuel R. Delany Has anyone actually read this nihilistic brick?
21. Dragonflight, Anne McCaffrey Worthy of love and rereading!
22. Ender’s Game, Orson Scott Card
23. The First Chronicles of Thomas Covenant the Unbeliever, Stephen R. Donaldson Waste of time and dictionary.
24. The Forever War, Joe Haldeman Influential boring waste of time.
25. Gateway, Frederik Pohl Ditto.
26. Harry Potter and the Philosopher’s Stone, J.K. Rowling
27. The Hitchhiker’s Guide to the Galaxy, Douglas Adams Should be much higher.
28. I Am Legend, Richard Matheson
29. Interview with the Vampire, Anne Rice Gothic vampires. Borrrrrring.
30. The Left Hand of Darkness, Ursula K. Le Guin
31. Little, Big, John Crowley Overrated.
32. Lord of Light, Roger Zelazny Zelazny has done much better.
33. The Man in the High Castle, Philip K. Dick
34. Mission of Gravity, Hal Clement A must for worldbuilding.
35. More Than Human, Theodore Sturgeon
36. The Rediscovery of Man, Cordwainer Smith Awesome! Fun, too!
37. On the Beach, Nevil Shute
38. Rendezvous with Rama, Arthur C. Clarke Eh.
39. Ringworld, Larry Niven
40. Rogue Moon, Algis Budrys
41. The Silmarillion, J.R.R. Tolkien Very good in places, very full of ideas.
42. Slaughterhouse-5, Kurt Vonnegut Almost total self-indulgent waste of time.
43. Snow Crash, Neal Stephenson
44. Stand on Zanzibar, John Brunner Not bad, actually.
45. The Stars My Destination, Alfred Bester
46. Starship Troopers, Robert A. Heinlein If you don’t read it, you can’t understand the gun-thread arguments.
47. Stormbringer, Michael Moorcock
48. The Sword of Shannara, Terry Brooks Sucks and blows like a tornado.
49. Timescape, Gregory Benford I can’t remember finishing this.
50. To Your Scattered Bodies Go, Philip Jose Farmer
Aliens in This World 
33 and 43 are pretty good – you should give ’em a try.
The Man in the High Castle especially. My brain keeps coming back to it when I’m trying to think about other stuff. Eve Tushnet had some posts awhile back about some theological ideas/parallels in it.