By far the best sci-fi book(s) that I have read is Hyperion and The Fall of Hyperion by Dan Simmons.
Those two books are really one book, but I think its hard to publish a 1,000 page sci-fi book this deep. The first half is written like a Canterbury Tales with a set of pilgrims going to see a demon from the future worhsipped by many as a god that moves backward through time, all set during a galatic civil war between the high-tech group of humans and those outside the high-tech sphere.
Several of the parts of the book, each characters pilgrim so far, are better than most books. Together, and how they fit together, is mind-stretching and simply a great piece of art.
I put it in my top three of pieces of literature that should stand for humanity's creativity: along with The Magus by John Fowles (without question the best piece of fiction I've read and something every RVFer should read) and 100 years of Solitude which is magical-historical literature as a genre.
If a person is into heavy, high tech, post human sci-fi with religious battles throughout, check out Hyperion and The Fall of Hyperion. Cant recommend more highly.
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Whats also disappointing is that I've read 100 years of solitude writer Gabriel Garcia Marquez's other big book Love in the time of Cholera and didnt think much of it at all. Same with John Fowles other big book The French Lieutenant's Woman and Dan Simmons several other novels. Even genius isn't limitless. A lot of amazing writers spend a great deal of their life with the one huge idea they have, and thats it. Its sad seeing it, hoping they could create a similiar thing again, but I just dont see it that often. Even Hemmingway's For Whom the Bell Tolls and Old man and the sea, His other long and short works arent very good.