Here are some of my favorites:
Childhood’s End by Arthur C. Clarke – Powerful aliens offer us peace and prosperity, but we must give up our dreams of going to the stars.
The Fountains of Paradise by Arthur C. Clarke – The challenges, setbacks and triumphs of building an orbital elevator or “beanstalk.”
All My Sins Remembered by Joe Haldeman – A spy for a distant-future republic begins to lose his own sense of identity the longer his career runs.
The Forever War by Joe Haldeman – Military sf novel about how relativity draws soldiers in an interstellar war away from their homeworld.
Tool of the Trade by Joe Haldeman – A Soviet sleeper agent in late 1980s Boston develops a practical form of mind control, and decides to use it for his own purposes as both the CIA and the KGB try to catch him.
Fatherland by Robert Harris - Chilling alternative history about a murder investigation in 1964 Nazi Germany.
Friday by Robert Heinlein – A courier/spy in the future is betrayed but decides to fights back.
The Moon is a Harsh Mistress by Robert Heinlein – Colonists on the Moon revolt against Earth’s harsh rule.
Starship Troopers by Robert Heinlein - A classic military sf novel about humanity fighting two alien races for survival.
Time for the Stars by Robert Heinlein – Telepaths are able to keep starships linked as we explore nearby worlds.
Fevre Dream by George R.R. Martin – An intriguing vampire novel set along the Mississippi River before the Civil War; the kind of book Bram Stoker and Mark Twain might have written together!
Tuf Voyaging by George R.R. Martin - Satirical sf novel about ecological engineering, overpopulation, absolute power and war. Probably my all-time favorite sf book.
The Time Traveler’s Wife by Audrey Niffenegger - The inadvertent and uncontrollable time-travelling of a young man wreaks havoc upon his decades-long romance with his eventual wife.
Old Man’s War by John Scalzi – Sardonic, clever military sf novel about humanity fighting several hostile alien races. There’ve been several very good sequels, too.