I haven't read much science fiction or fantasy. During the years when I might otherwise have been reading a lot of science fiction, I was instead reading superhero comics. In fact, I can fairly easily list all such books (relatively loosely defined) I have read:
The Lord of the Rings by J.R.R. Tolkien - Perhaps you've heard of it? I've read this twice, once when I was 12-13, and then again when I was 24. I enjoyed it both times. Given how often my father and especially my brother have re-read it, and with the recent movies, I've leafed through it a few times to see whether I might want to give it another go, but the prose just doesn't do it for me anymore.
Childhood's End by Arthur C. Clarke - I've also read this twice, once for a ninth grade book report, again in my early 20s. I thought it was damn good.
Dune by Frank Herbert - I read this around the same time I re-read The Lord of the Rings, and I remember loving it. My dad keeps getting on me to read the other books in the series, but I don't see it happening.
Several novels by Kurt Vonnegut, including Slaughterhouse-Five, Cat's Cradle, Slapstick, and Timequake, which seem to be the most "science fictiony". I don't have Vonnegut in the highly rigorous "science fiction" column in my mind, but I know that he is talked about a lot as such, so I include him here. The first two of these novels are my favorite of his overall. Timequake had an interesting premise that he did nothing with.
Something Wicked This Way Comes & Fahrenheit 451 by Ray Bradbury - These were ok, not terribly exciting.
Foundation by Isaac Asimov - I borrowed the original Foundation trilogy--along with the Bradbury, as well as the Card and Finney novels mentioned below--from my dad in an effort to read some of the classics of the genre, but I was unable to get past the first one; I found it virtually unreadable.
Stranger in a Strange Land by Robert Heinlein - probably the worst novel I have ever read.
Good Omens by Terry Pratchett & Neil Gaiman - this book is hilarious, and I wish I knew what happened to my copy; I keep meaning to read some Pratchett proper and, in fact, just gave a couple of his books to my brother for Christmas, to some acclaim, so I think I'll need to snag them from him.
Neverwhere by Neil Gaiman - I loved Gaiman's Sandman comics, so I was at least interested in his novels. This one, I just learned from Wikipedia, is apparently adapted from a BBC series he wrote. Anyway, the book was ok.
Red Dwarf: Infinity Welcomes Careful Drivers & Red Dwarf: Better Than Life by Grant Naylor - Ha! These are very entertaining and very silly, though I didn't realize until later that the novels came after the even sillier BBC series, which I didn't see until recently anyway (I liked the books better). "Grant Naylor" is, of course, the name under which Rob Grant and Doug Naylor collaborated.
Icehenge, Antarctica, & Red Mars by Kim Stanley Robinson - I was quite impressed by Robinson's novels (I thought his evocation of the cold in Antarctica, for example, was very effective), even though they did occasionally get bogged down in the science, and I fully expected to read more of his stuff, but, alas, have not as yet. Icehenge was the first science fiction I'd read that had a kind of literary uncertainty, if that makes any sense (and it might if I troubled myself to elaborate).
Ender's Game, Speaker for the Dead, Xenocide, & Children of the Mind by Orson Scott Card - The first Ender series. Ender's Game is enjoyable, if problematic. Speaker for the Dead was excellent. Midway through Xenocide he kind of lost me, and Children of the Mind didn't leave much of an impact on me at all.
Time and Again & From Time to Time by Jack Finney - I admit that I'm something of a sucker for time travel stories (though I've not actually read many), including shit like Back to the Future. Time and Again is an interesting twist on the idea: characters think themselves into the past (so long as they are in a certain location, if I recall correctly). Pretty good. The sequel is inevitably not quite as good, but still it held my interest.
Light by M. John Harrison - I first heard about Light via The Complete Review; I was duly impressed and have turned others onto it, who've also enjoyed it. I look forward to reading more of his fiction--for example, this Matthew Cheney post has interested me in The Course of the Heart.
Stranger Things Happen & Magic For Beginners by Kelly Link - various bloggers have been gushing about Link and her short stories for the last few years, so I was intrigued. I read Magic for Beginners first, and I understood why. Readers seem to think that her earlier collection was even better, but I disagree. I was not nearly as enchanted by the stories in Stranger Things Happen.
Parable of the Sower by Octavia Butler - see my review here
Little, Big by John Crowley - A great book. My first Crowley. I look forward with great interest and anticipation to reading his Ægypt tetralogy, once the fourth is finally published (scheduled for later this year) and the first three come back into print.
Ok, I think that's about it. No Philip K. Dick (interested, in theory, but wary of what is, by most accounts, often dreadful prose). No Samuel Delaney (who I definitely do plan to read). No Ursula Le Guin. No Handmaid's Tale (though I have read other Margaret Atwood novels). Not included: novels by Pynchon, Delillo, or Powers, etc, that might appear to have some elements of what could be called science fiction; so-called "magical realists" like Garcia Marquez or Rushdie; fabulists like Borges or Calvino. And so on.
All of this is to clear away the decks and get to the point, which is that very recently I did read a great science fiction/fantasy series: Gene Wolfe's The Book of the New Sun tetralogy, and its sequel, The Urth of the New Sun. I plan to write a little bit about it soon.