Gotta suggest Vonnegut. Not a typical sci-fi guy, since he used science fiction conventions to hide his social satire, but still counts. Slaughterhouse Five and Cat’s Cradle are possibly his masterpieces, though I’m also a fan of the under-appreciated Galapagos and the short-story collection Welcome to the Monkeyhouse.
Any particular work you’d suggest?
I would love an excuse to read Catch-22 or Cat’s Cradle
Catch-22 is Joseph Heller. Great book, though. (Even better on re-reads too.) As I mentioned above (ehem…Chuck), I’d suggest starting with Slaughterhouse Five, Cat’s Cradle (perhaps the funniest book about the end of the world, ever) and the short story collection Welcome to the Monkeyhouse. (There’s apparently a “right” order to reading some of Vonnegut’s books, particularly the ones that feature Kilgore Trout as a main character, but I’ve never figured it out.) I also have a soft spot for Galapagos and Breakfast of Champions, though I can’t recall the story of the latter at all, just the bawdy little drawings scattered throughout the book.
Cat’s Cradle is really cool, but my all-time favorite is Sirens of Titan. There’s no excuse for a science fiction fan NOT reading this at some point.
Slaughterhouse Five is a great book, though it’s not as sci-fi as I imagined when I heard it labeled sci-fi. It’s kind of history, psychological dreamscape, modernist literature…with a little sci-fi thrown in. Certainly a very influential book. I did like it, though I found it somewhat complex and a little disturbing.
The thing I find about Vonnegut and sci-fi is that it’s really sci-fi by convenience. The genre provided Vonnegut with a convenient mask under which to hide his curmudgeonly social satire in the 50’s, 60’s and 70’s. It’s interesting (and telling?) that after Watergate (okay, a while after Watergate, since his last novel is from the 1980’s) he never wrote another work of fiction but concentrated instead on small essays and commentaries for various journals as well as NPR.
Tell that to the Tralfamadorians!
Incidentally, though his work isn’t all sci-fi oriented, Tralfamadore is one of the few common threads between many of his novels.
I’d be happy to read or reread any one of his.
Slaughterhouse 5 or Cat’s Cradle would be good choices I think. Cat’s Cradle would also fit in nicely with the discussions of religion, in which Armando and I have been participating!
Galapagos is also a good one - our big brains as an evolutionary disadvantage.
I don’t think we can go far wrong with Vonnegut.
While we’re on SF books, you might want to check out the podcast at www.starshipsofa.com - a couple of real characters dealing each week with a particular SF author. The 'cast is idiosyncratic to say the least, but great fun, and - Chuck et al - the guys who do it would relate to your “p******mate” blooper 
Plus they’re great fans of tatty old paperbacks found in secondhand book stores and charity shops. Their podcast has introduced me to some great authors I’d never been aware of. The podcast on Harlan Ellison is particularly good.