So much good stuff out there!
Hands down, my all-time favorite sf book is George R.R. Martin’s Tuf Voyaging, a terrific collection of interrelated short stories about the misanthropic, sarcastic captain of a massive starship with very impressive ecological engineering and cloning capabilities. Great tales that tackle some interesting issues, including absolute power, overpopulation, environmentalism and militarism. I’ve often recommended the book here on the boards, and without exception everyone who’s read it has raved about it. Hope you like it, too.
Arthur C. Clarke’s The Fountains of Paradise is a very interesting look at building a space elevator or “beanstalk.” Also very good by Clarke: 2001: A Space Odyssey, 2010: Odyssey Two, and Childhood’s End.
John Scalzi’s Old Man’s War is the first book in a series, about humanity fighting a number of hostile alien races. It’s right up there with Robert Heinlein’s Starship Troopers and Joe Haldeman’s The Forever War for hard, page-turner military sf, but with more snark, humor and snappy writing.
Larry Niven and Jerry Pournelle’s The Mote in God’s Eye is one of the best-ever alien first contact stories, IMHO, set in the distant future when humanity’s starfaring empire is climbing back up after a long fall.
Frank Herbert’s Dune is a deserved classic, about a desert world with a hidden subculture at the center of galactic imperial politics.
Isaac Asimov’s The Robots of Dawn is my favorite book of his, but I, Robot and The Foundation Trilogy are also well worth a read, although slightly dated now.
Heinlein’s Time for the Stars, about near-FTL travel and ESP, is my favorite of his many books, although Have Space Suit - Will Travel, Glory Road, The Moon is a Harsh Mistress, Space Cadet, The Rolling Stones, The Puppet Masters, Friday and the aforementioned Starship Troopers are all excellent.
Ray Bradbury’s The Martian Chronicles and The Illustrated Man are his best, I think - collections of just one amazing short story after another.
The Left Hand of Darkness by Ursula K. Le Guin is an engrossing tale of a human diplomat trying to learn about a very different alien culture with shifting sexual identities.
Alan Dean Foster’s Icerigger is, I believe, his best, about a small group of humans stranded on a wintry alien world who board a great ship that sails the frozen seas.
Joe Haldeman’s The Forever War is, as mentioned above, a great military sf novel. My favorite of his, though, is Tool of the Trade, a Cold War sf/espionage thriller about a Soviet sleeper agent in the U.S. who develops a practical method of mind control. A great story with a perfect ending.