That’s what I was going to recommend - it took me far too long to get around to this book, and it is fantastic. I didn’t just read it; I relished it.
I’m glad it’s not just me who does this. 
I do love short stories, though - perfect for a before-bed read.
Asimov wasn’t great to start with, and hasn’t aged well. However, as others have said, “The Gods Themselves” is a fine novel and well worth reading.
It wasn’t clear from the OP: did you read Dune, or did you come into the series at Chapterhouse? If you were just going to read one, the first one is definitely the one to read.
I quite like a lot of Azimov’s stuff. I’d say The Gods Themselves is a better start than Foundation. I’d also recommend Nightfall (the short story) if you can find it.
As for Niven, I loved his exploratory essays and the early known space stuff. His “Gil the ARM” series is good, too. His essay, Man of Steel, Woman of Kleenex, had me quite literally laughing out loud. Is Ringworld Engineers the only one of that series you’ve read? I think the first Ringworld book was better. I don’t remember the first story that introduced Pak Protectors, but it’s a unique concept in SF.
I’d agree with the Dragonriders of Pern recommendation. Do the first trilogy and decide whether you want to keep going. It is science fiction that reads like fantasy, though, so if you’re looking for hard SF, this probably isn’t your thing.
Even though it’s dated, see if you can find The Complete Venus Equilateral by George Smith. Great early science fiction with plenty of science (well, mostly tech, but that counts).
My favorite Heinlein book is Time Enough for Love. It’s a massive tome, but it’s well worth it.
Where to start with Spider Robinson? His stuff is all over the map. I’d say read Time Travelers Strictly Cash. If you don’t find anything in there you like, you probably won’t like any of his.
Bester, Brin and Banks.
and Le Guin and Cherryh.
I love Time Enough for Love, but it might be a little confusing if you have not already read Methuselah’s Children. And Methuselah’s Children might be a little confusing if you have not already read Revolt in 2100.
I would also recommend Space Cadet and The Rolling Stones (the latter because it features the original tribbles, and Hazel from The Moon is a Harsh Mistress as a curmudgeonly old grandmother.)
I would also recommend any collection of Harlan Ellison short stories. He was doing twist endings long before M. Night Shyamalan made it a joke. Also read some of his essays. He may be a jerk, but he is a highly entertaining jerk.
[quote=“Gary “Wombat” Robson, post:43, topic:666218”]
It wasn’t clear from the OP: did you read Dune, or did you come into the series at Chapterhouse? If you were just going to read one, the first one is definitely the one to read… His essay, Man of Steel, Woman of Kleenex, had me quite literally laughing out loud. Is Ringworld Engineers the only one of that series you’ve read? I think the first Ringworld book was better.
[/QUOTE]
Yes, the “-” above is meant to be “through” and the “,” is “and”. Neither is “,the series, of which I’ve read only these:”
It would be a strange read indeed to come into Dune starting with Chapterhouse! :eek:
I’ve also read Man of Steel, Woman of Kleenex, as every good comic geek should.
Heh. I tend to think of “classic” SF as “anything I read as a minor”, that is, anything that was published before the mid 70s. My grandfather had an extensive SF library, and he let me browse through most of it, so I was pretty heavily influenced by his tastes. If he bought something and regarded it as too modern, he’d usually pass it on to me, though, so I wasn’t completely overwhelmed by Engineer Porn.
James P. Hogan has written some gems. The *Giant Star *series is amazing, as is the *Code of the Lifemaker *series.
Some of the material is a little dated, e.g. the USA relationship with “The Soviets,” but that is a minor detail. Real page-turners.
Absolutely read Downbelow Station by CJ Cherryh.
And the best sci-fi parodies ever, Bill the Galactic Hero and Star Smashers of the Galaxy Rangers, both by Harry Harrison.
Also by Harrison – the Deathworld trilogy, which Spielberg ripped off as the basis for that fuckin’’ awful Avatar, and is about a thousand times better than it.
Datedness is not an issue for me. Heck, if it’s dated, it’s classic almost by definition, isn’t it? 
After all, I’m enjoying Lensmen, for goodness sake, and it reads like a sci-fi film noir, complete with brassy dames and strong-chinned tough guys. 
And as mentioned, I love The Doomsday Book forever and ever… even a huge portion of the plot could be avoided if Willis had predicted the ubiquity of cellphones.
Julian May’s four-book “Saga of the Exiles”, “Intervention”, and most importantly her “Galactic Milieu” trilogy.
Some of the greatest characters in sci-fi history, as shown by a wonderful storyteller, in a tale that lasts six million years.
Atoning Unifex and “Jack the Bodiless” FTFW.
I highly concur with the recommendation - the second book is especially outstanding - but if there’s a connection to Avatar I am completely missing it.
And I don’t think Spielberg had anything to do with Avatar. 
Stainless Steel Rat - Harry Harrison, Deathworld was good too +1
Martian Chronicles - Ray Bradbury
Non-Stop (Aldiss)
I don’t know how I forgot that one. It would drive the pure science fiction crowd crazy, I suppose, but I love the crossovers, and this has as much noir tough guy in it as it does science fiction.