I second the nomination of Cat's Cradle as sci-fi. It's got even more elements of the stuff than Slaughterhouse-Five, which kind of was even though it didn't want to be.
Now, for unmentioned ones:
Armor, by John Steakley: This is what Starship Troopers would be if instead of focusing on the political, it looked at one person and the effects of a future war on his poor little mind. It also has a wonderful plot arc that seems to go all over the place but, in reality, is neatly connected.
The Mote in God's Eye, by Larry Niven and Jerry Pournelle: Humanity's first contact with really alien aliens. With lots of very realistic politics and double-dealing. Dialogue isn't the best, but that does not detract from the overall quality.
First Contract, by Greg Costikyan: If aliens were to integrate Earth into the galactic economy...well, this might occur. Wildly amusing, and just as interesting to read.
Altered Carbon, by Richard K. Morgan: The definitive cyberpunk novel of the 21st century. No, really. This guy gives Gibson the run of his life for his accumulated moneys. He manages to make a novel where people who die can just be resurrected thrilling and attention-grabbing.
Agreed on Reynolds, Gibson, Herbert, etc...though I have mad beef with Arthur C. Clarke. I also do not like Dan Simmons. He wrote a wonderful story of interlocking tales and then at the end he hit it with a hammer marked PERSONAL POLITICS. And that made me very sad. Then he went insane and made the Saga of the Evil Catholic Church #392.
...and the Space Trilogy. Oh, I remember those. I read the end of That Hideous Strength and I wanted to find that dead man and light him on fire. And then light the ashes on fire. And so on.
For a dead man I actually like, however...plenty of people have referenced "-ian" things of his, but Howard Phillips Lovecraft is a sci-fi author indeed, good sirs/madams, and one of some repute in my book.