Based loosely on the sub genres in the Wikipedia entry for SF, here are good starting points for those who want an introduction to the field. Not intended to be a comprehensive list of recommendations. Some are contemporary, some older novels:
Hard SF: Alastair Reynolds, Revelation Space series
Soft & Social: Ursula K LeGuin, the Left Hand of Darkness; Gene Wolfe, the Fifth Head of Cerberus
Cyberpunk: William Gibson, Neuromancer; Neal Stephenson, Snow Crash
Time Travel: Kurt Vonnegut, Slaughterhouse Five
Alternate History: Philip K Dick, The Man in the High Castle
Military SF: Joe Haldeman, The Forever War
Superhuman: Theodore Sturgeon, More than Human; Olaf Stapledon, Odd John
Apocalyptic: Walter Miller, A Canticle for Liebowitz; Cormac McCarthy, The Road
Space Opera: Iain Banks, The Algebraist; Alistair Reynolds, Revelation Space Series
Steampunk: William Gibson and Bruce Sterling, The Difference Engine
New Weird: China Mieville, Perdido Street Station
Science Fantasy: Iain McLeod, The Light Ages
End of Time & Far Future: Gene Wolfe, Book of the New Sun, Book of the Long Sun, Book of the Short Sun; Jack Vance, The Dying Earth; Cordwainer Smith, Collected Stories
A Few Notes:
Gene Wolfe and Cordwainer Smith are both Roman Catholic authors, and though their faith informs their work, they don’t clobber you over the head with it (as is the case with C S Lewis’ space fiction).
Science Fantasy works out what life would be like if a fantasy element were real. In The Light Ages, for example, aether is a magical substance that is a source of power and can be mined, though with negative health results for the miners, turning them into changelings.
Jack Vance’s Dying Earth informed or inspired Gene Wolfe’s epic set of nine or so novels set in the far distant future.