My favorites, after some 50 years of reading sf are:
1.)
The Classics, mostly "hard sf"
Isaac Asimov (for his "Foundation Trilogy" and his "Three Laws of Robotics", if for nothing else, but his works are legion)
Robert Heinlein (especially the books and stories in his "Future History")
Poul Anderson
Robert Sheckley
Roger Zelazney (His short story, "A Rose for Ecclesiastes" is, IMO, the finest short story in the English language.)
Harry Harrison, the "King of Alternate History"
S.M. Stirling
Andre Norton
C.M. Kornbluth
Keith Laumer
Phillip K. Dick (especially his
Man In A High Castle)
2.)
More Modern authors, including a good bit of fantasy:
Neil Gaiman (especially
American Gods,
Nerverwhere, and the "Sandman Series" of graphic novels)
Ursula Le Guin
Stephen Lawhead
Jack Whyte (for his "Camolud Chronicles")
Bernard Cornwell (for his "Winter King Trilogy" on King Arthur)
Fred Saberhagen (for the whole "Berserker Series" from which we get the Death Star in "Star Wars" and the Cylons in "Battlestar Galactica", except that Saberhagen's robotic killers are truly scary. He also does horror genre in his "Old Friend of the Family" series about Dracula as a, more or less, hero.)
3.)
Some space-opera authors:
E.E. (Doc) Smith
Edgar Rice Burroughs(perhaps more fantasy, I don't know)
A. Bertram Chandler (for his great John Grimes stories, for more, see
http://www.bertramchandler.com/)