Geez, I could recommend a hundred books off the top of my head. What are some books you liked?
In no particular order...
Footfall and The Mote in God's Eye, both by Jerry Pournelle and Larry Niven. Also Lucifer's Hammer, though I liked the other two more.
The first couple books in Niven's Ringworld series are pretty good.
Century Rain by Alastair Reynolds.
Larry Correia's Grimnoir Chronicles series is excellent. Pretty much everything Larry writes is awesome but those are more in the fantasy genre than his other books.
Nick Sagan's Idlewild series is good, and strange.
S.M. Stirling writes some good stuff. The Peshawar Lancers and Conquistador are my favorite books of his; they're both standalone novels. Neither book fits into the scifi/fantasy genre well, actually, but many of his other books do.
Heinlein's works are worth exploring of course. Glory Road is my favorite. However, I didn't even finish Stranger in a Strange Land or a few of his other more popular books.
Jim Butcher's Dresden Files series is worth checking out.
Jack McDevitt's Academy and Alex Benedict series both start out really well, but go downhill a few books in. Still, they're essentially standalone books and worth reading (but better in order). I think the Academy series is good through Odyssey, and the Alex Benedict series is good through Seeker. I also liked Eternity Road, which is not part of a series.
Joe Haldeman's The Forever War.
Scalzi's Old Man's War series.
Armor, by John Steakley. Also, Vampire$.
Pretty much everything by Michael Crichton is worth reading. Jurassic Park is notable for being ten times better than the movie.
Robert Rankin's The Hollow Chocolate Bunnies of the Apocalypse is strange but very amusing. I don't know about this rest of his books.
Terry Pratchett must be on the list, specifically the Discworld books. I like the Witches and City Watch storylines the best.
Gil's All Fright Diner, Divine Misfortune, In the Company of Ogres, and The Automatic Detective by A. Lee Martinez.
Snow Crash, by Neal Stephenson. I haven't read the rest of his books recently enough to recommend them.
Charles de Lint's The Little Country and Someplace to be Flying.
Sarah Hoyt's Darkship Thieves.
Dan Simmons' Hyperion Cantos. I didn't think it was anything too special, but lots of people disagree with me.
I don't have my book collection here or I would have many more recommendations, but that should do for now.
Edit - The Empire of Man series by John Ringo and David Weber. Can't believe I forgot that.