Philip K. Dick is my all-time favourite, I own pretty much everything he has written. Gollancz has published most of his short stories in 5 volumes sorted in chronological order (
Beyond Lies the Wub,
Second Variety,
The Father-Thing,
Minority Report, and
We Can Remember It For You Wholesale), and as PKD was an excellent short-story writer, those are definitely worth checking out. To name a few of his novels that I've liked the most, there's
Ubik,
A Scanner Darkly, and
Lies, Inc. VALIS is also an excellent read, though I'm not sure I'd classify it exactly as science fiction.
Iain M. Banks was also a great writer, and I've been quite saddened by his recent death.
Excession is probably my favourite, I really like the scheming AI-ships of the Culture.
A few others I've enjoyed recently:
Incandescence by Greg Egan. I've not really a fan of his other works that I've read so far, but this one worked well.
Altered Carbon by Richard Morgan. In similar style, Neal Asher's
Gridlinked was also good.
The Diamond Age by Neal Stephenson. Another favourite writer of mine; I'd grade his
Cryptonomicon as possibly the best book I've read (though it's not sci-fi).
I have a smallish sci-fi collection (about 500 books, out of my total collection of about 1100 books). After buying all the obvious classics of the genre, I've found
http://www.bestsfbooks.com a valuable resource for finding possible candidates.