I read a lot of sci fi in HS, all the classics. Dune and Asimov's Foundation trilogy and the Robot series are great books in any genre. I don't read as much of it now, but some books stick in your mind and refuse to be left behind—
The Lathe of Heaven—Ursula K. LeGuin
Stranger in a Strange Land,The Cat Who Walked through Walls—Robert Heinlien
The Amber series-starting with Nine Princes in Amber, I believe, by Roger Zelazny—also that crazy one he wrote about the Hindu gods-Lord of Light ??—title escapes me atm…
Slaughterhouse Five, Welcome to the Monkey House, God Bless You, Mr. Rosewater, Player Piano—Kurt Vonnegut
Of newer stuff, I recently read The Algebraist, by Iain Banks—reminiscent of Vonnegut to me, and very good.
And a few sci-fi fantasy hybrids—
I always recommend Valerie Leith's Company of Glass trilogy—a bit convoluted at times but some interesting if occasionally long-winded philosophy under the sci-fi fantasy hood.
I like Barbara Hambly's older stuff, too—the Darwath series including Icefalcon's Quest, Mother of Winter,etc, the standalone Bride of the Rat God (a bit satirical) and the Dragonsbane ones, though Dragonshadow, the last, is extremely depressing.
Death of the Necromancer—Martha Wells—great theivery and high fantasy, pretty well written.
The Bone Doll's Twin—Lynn Flewelling—great dark fantasy with some occasional gruesomeness, also pretty well written. This is part of a series but I've never caught the rest of it.

But these are closer to fantasy, not sci-fi.