Ender's Game by Orson Scott Card
Children trained to command space forces against giant bugs. Twists! Also highly recommend its sequels Speaker for the Dead and Ender's Shadow. More great stuff from Card is Treason, Pastwatch, and The Worthing Saga.
Hyperion by Dan Simmons
Six interesting people come face to face with a mysterious entity on a strange planet. So cool.
Starship Troopers by Robert Heinlein
Be-suited space marines train to kill giant bugs. Heinlein is great, I also really like Tunnel in the Sky and The Moon is a Harsh Mistress.
Wool by Hugh Howey
A great series that is like City of Ember for adults. People live in a large underground silo but aren't quite sure why.
The Day of the Triffids by John Wyndham
Strange alien plants knock mankind back to the Stone Age. Will they survive?
The Time Machine by H.G. Wells
A man goes to the future and finds... something not good.
The Mote in God's Eye by Niven and Pournelle
A great story of encountering an alien culture with a lot of ethical questions.
Replay by Ken Grimwood
Not sure that it's sci fi exactly, but a man gets to live out his life over and over, making different decisions each time. In the same vein, The First Fifteen Lives of Harry August by Claire North and A Gift of Time by Jerry Merritt were super fun.
Planet of the Apes by Pierre Boulle
A man travels across space to an all-simian society. Not a lot like the movie.
Childhood's End by Arthur C. Clarke
Mankind is on the verge of learning a terrible truth that's been kept from them. I also like Rendezvous With Rama.
Earth Abides by George Stewart
Most of the population is killed off. A simple survival story in the aftermath.
I was a big Bantam-era fan as a kid, and this one is my favorite. Luke hadn't become a boring guru yet and the sci fi threat would be cool even if it wasn't in the Star Wars universe.
Ready Player One by Ernest Cline
Get ready for pop culture overload in a wish-fulfillment future video game. Guilty pleasure, I suppose.
The Host by Stephanie Meyer
Survival in the midst of a body-snatching alien invasion. Guilty pleasure, I suppose.
The Forever War by Joe Haldeman
A brisk, fun account about the downsides of futuristic war.
A brisk, fun account about the downsides of futuristic war.
Some others I've liked quite a bit include...
Sci Fi
Sci Fi
The Left Hand of Darkness by Ursula Le Guin
The Hitchhiker's Guide to the Galaxy by Douglas Adams
Old Man's War by John Scalzi
Down and Out in the Magic Kingdom by Cory Doctorow
The Giver by Lois Lowry
Terms of Enlistment by Marko Kloos
A Fire Upon the Deep by Vernor Vinge
Post Apocalyptic
Lucifer's Hammer by Larry Niven and Jerry Pournelle
Post Apocalyptic Nomadic Warriors by Benjamin Wallace
Cthulhu Attacks by Sean Hoade
Life as We Knew It by Susan Pfeffer
The Road by Cormac McCarthy
Terms of Enlistment by Marko Kloos
A Fire Upon the Deep by Vernor Vinge
Post Apocalyptic
Lucifer's Hammer by Larry Niven and Jerry Pournelle
Post Apocalyptic Nomadic Warriors by Benjamin Wallace
Cthulhu Attacks by Sean Hoade
Life as We Knew It by Susan Pfeffer
The Road by Cormac McCarthy