If you like military science fiction, the Hammer’s Slammers series by David Drake is a classic.
In addition to agreeing about Isaac Asimov, I also suggest several books by Sherri S Tepper: The Fresco, Gate To Woman’s Country, The Visitor, Family Tree, and a couple others I can’t think of right now.
And also L.E. Modesitt Jr, Andre Norton, and C.J. Cherryh.
I’ll third (or fourth, or whatever we’re up to now) Heinlein’s Moon is a Harsh Mistress. IMO, it’s his best work, although Stranger in a Strange Land and Double Star are also excellent. You’ll get conflicting opinions about Starship Troopers, but I enjoyed it. Those four would be Heinlein’s four Hugos. Other good choices from Heinlein might be Farmer in the Sky or Time Enough for Love. If you enjoy Moon is a Harsh Mistress, you might consider The Rolling Stones, just to see a shameless reusal of a character.
I loved Asimov’s Foundation, but I thought the sequels went downhill fast.
Larry Niven and Jerry Pournelle often collaborate on books, usually with excellent results. Footfall, The Mote in God’s Eye, Lucifer’s Hammer, and in collaboration with Stephen Barnes, The Legacy of Heorot. That last is one of the best colonization stories I’ve ever read.
I’d also vote for Pohl’s Gateway and Le Guin’s Left Hand of Darkness.
And Etumretniw is right, Ender’s Game is one of the greatest sci-fi novels in existence. The sequels are also good, but in a much more thoughtful and much less dramatic way.
Neal Asher writes cyberpunkish space opera set in an improbably hostile universe. I recommend **‘Line of Polity’, ‘Gridlinked’ ** and ‘Brass Man’. The last two feature Mr Crane a psychopathic android hitman: - “They called him Crane because he was so very tall. They called him Mr Crane because he was so very prone to dismembering people.”
I haven’t seen anyone in this thread mention William Gibson. I’m not big into sci-fi (or at least the hardcore stuff), but it seems like every big-wig sci-fi geek recommends him - I can’t get through a bloody issue of Wired without someone singing his praises. How does he stand up with the sci-fi readers on this board? I started reading Idoru but I gave up partway through, it was too boring.
Well, kushiel, I thought Gibson’s Neuromancer was great, but Idoru didn’t do much for me, either.
Johnny Angel and other Retief fans, you do know that there’s a new sequel out called Retief’s Peace, right? The author is the same fellow who continued Keith Laumer’s Bolo series.
I’d also recommend Sarah Zettel’s books. I’ve found Fools War and Playing God interesting and thought-provoking. In other words, SF at its best!
CJ
Thanks for this thread. I’m almost done with A Storm of Swords(Fire & Ice book 3) and am too cheap to get Feast of Crows in hardcover. Thought I’d break up my fantasy streak (the other Fire Ice books) and read some SF. I couldn’t decide between Peter F. Hamilton’s Reality Dysfunction, Part 1: Emergence and Tad Williams’ Otherland Vol 1. (of the books recommended here that I have not read and I could find these two appealed to me the most)
so I bought both 
Brian
Lotsa good stuff named here!
I’ll put in my obligatory plug for the works of E. E. “Doc” Smith, the Lensman series, and the Skylark series.
The first book in the latter series, Skylark of Space, as it was one of the first, if not the first, novel about interstellar travel, published in 1928.
Oh, yeah! When I put in my plug for the Lensman series a few posts back, I completely forgot about the Skylark books. I think his characters were prototypes for characters in a lot of later science fiction movies.
You don’t exaggerate even slightly! 
Didja ever see the “Doc” Smith sample in the classic thread if LotR Had Been Written By Someone Else!? It’s post #15 (of over 2000) in the thread.
Do you know Periwinkle? 
How in Middle-Earth did I miss that thread? It’s wonderful. It’s incredible. It’s too darned long. And your Smith fits it beautifully.
Periwinkle?
Sorry for the hijack,
but what’s the difference between ‘hard’ and ‘soft’ sci-fi?
I’m sure several replies will hit before mine precipitates, but:
“Hard” science fiction is fiction where the science (mostly) accurate, and is often crucial to the plot or the point of the story. Edgar Allan Poe and Jules Verne arguably started this form. Other notables are Robert Heinlein, Arthur C. Clarke, Larry Niven, Isaac Asimov, the late Hal Clement, and a great many others. It does not imply a lack of interest in characterization, human values, etc., although it is certainly possible for a writer to get too obsessed with his gadgetry, just as any writing can have faults.
“Soft” science fiction is fiction where this is not the case. Lots of excellent SF writers didn’t try to get into details, concentrated on other aspectsd of the story, or sometimes didn’t know a lot of science. Robergt Sheckley, Fredric Brown, Jack Chalker… there are plenty of examples. Fredric Brown’s scientific errors are sometimes outrageous, but he was a helluva writer.
In the grey area we’ve got people whose science fiction is so far out that it’s not really meaningful to call it “hard” or “soft”. “Doc” Smith’s space opera space ships are so out-of-there that there’s no point in trying to call them oner way or the other. (But when he writes about, say, mining, he’s got his techn ology accurate). Is anything of Dan Simmons’ Hyperion series remotely like achievable technology? Who can say?
Or you’ve got “soft” sciences like Anthropology. Controversial territory here. Is Ursula K. LeGuin “hard” or “soft”? Just because it’s Anthropology instead of Nuts and Bolts doesn’t mean it ain’t science.
I don’t care, yours was awesome. Thanks! mwah
You know! Wombat and Periwinkle! “Doc” Smith’s crack acrobatic/espionage team from Imperial Stars! I thought with your handle, you must have read about them.
:smack: