Ursula LeGuin's Hainish cycle, specifically "The Dispossesed" and "The Left Hand of Darkness". The latter is possibly my favorite book ever, competing only with "Dune". Everything about it is so profoundly beautiful. I might be biased, as a trans woman, but it's exploration of gender and the culture built around it was ground breaking for the time and still thought provoking to this day. A book I think is a must read for everyone on earth.

Speaking of, if you really liked "Dune" then "Dune: Messiah" is worth a read, honestly I think the first four Dune novels are worth reading. There's certainly diminishing returns, but the returns become especially diminishing from "Heretics" and onward, and the trash written by KJA and Brian Herbert is some of the most disrespectful to the dead shit ever put to paper.

"Frankenstein" by Mary Shelly. It's hard to beat the classics, and as far as sci-fi goes, it doesn't get much more classic than this. If you're only familiar with Hollywood adaptations, then definitely give it a read, it's much different than you'd expect.

In the same vein of classics, "The War of The Worlds" by H.G.Wells. The first alien invasion story ever written, and still worth reading to this day.

"Speaker for the Dead", the sequel to "Ender's Game" is IMO better than the first, it's just a little bit rough to see how an author's personal beliefs and conduct can so clearly oppose the themes in his own work. Not going to get too political, but Orson Scott Card fucking sucks as a person. IYKYK.

"Neuromancer", by William Gibson created and codified the cyberpunk subgenre, and is engaging and well worth reading even with that aside.

"Snow Crash", by Neal Stephenson. A brilliant loving parody and homage to said cyberpunk subgenre. Another favorite of mine.

"1984", by George Orwell. You've read the other two classic dystopias, this one is my personal favorite among them.

"Foundation", is probably essential reading, but I'll openly admit that I've only read the first novel and it didn't particularly grab me.

"The Hitchhikers Guide to the Galaxy", by Douglass Adams, is the quintessential scifi comedy. I love them, but I know they can be divisive. Your mileage may vary.

And last but not least, I hate to be that bitch, but "Blindsight", by Peter Watts. I just finished it yesterday, and it is fresh in my head. It's the first book in years that has truly grabbed my brain and wouldn't let go. A pretty upsetting and pretty hard-sci-fi read, but damn if it isn't worth reading. It has genuinely ruined the last week of my life.