Single Recommendations by category....I do not have anything new to surprise anyone, just my take on older stuff.
Serious Essential Classics! Great Reads, strong philosophical relevance for readers regardless of interest in Sci-Fi
[1] Farenheit 451, Ray Bradbury, 1953
[2] Nineteen Eighty-Four, George Orwell, 1948
[3] Brave New World, Aldous Huxley, 1932
[4] A Canticle of Leibowitz, Walter M Miller Jr, 1960
Foreign Original
[1] Solaris, Stanislaw Lem 1961
For western Sci-Fi readers brought up on Asimov, Niven, Pournelle this is definately a nice departure from stereotypical "aliens" in most novels
Computer Age/AI/Singularity
[1] Neuromancer, William Gibson 1984
[2] Count Zero, William Gibson 1986
...See also Dan Simmons Hyperion series
[3] Verner Vinge: have not read but feels like great fit here.
Essential Authors
Authors.....where series or complete works are worth dipping into and exploring
[1] Dan Simmons: Hyperion Series
[2] Issac Assimov: Robot Series, Foundation Series
[3] Frank Herbert: Dune series
[4] Arthur C Clarke: All Series
Classic
[1] Dune, Frank Herbert, 1960
[2] Phillip K Dick: All, dark and visionary
[3] Ray Bradbury: lots more
[4] Richard Matheson; lots
Lighter *fun* Reading
[8] Larry Niven: Excellent reads most stories/novels. Great fun and Interesting Ideas, Worlds, creatures and Universe
[9] Kim Stanley Robinson: (Mars Trilogy) Excellent, political ecological intrigue
[10] Jules Verne: Classics, just make sure to find good translations or read French Originals
Fantasy
J.R.R Tolkien Masterpieces for all ages, a world, a history, lore and creation woven into reality
David Eddings Creative, fun and lightweight
George R R Martin A Song of Fire and Ice series (Game of Thrones) , forget the show, books are superb