Well, everyone knows Sturgeon’s Law, I think. And it applies to what gets published as Science Fiction quite as much as anything else.
Here would be my list:
Robert A. Heinlein: Though it’s easy to nitpick much of his output, he essentially created the modern genre as a literary form. Everything he wrote is worth reading, except perhaps his never-published-until-2003 first novel and the “three stinkers” – and criticisms of it are somewhat akin to finding fault with Hamlet or Romeo and Juliet because “it’s full of cliches” – he got there first.
Arthur C. Clarke: Clarke’s work is deceptively “smooth” but contains some deep insights, and is nearly always a good read. Clarke’s collaborations with Gentry Lee are an acquired taste, and most fans of Clarke’s noncollaborative work seem not to like them.
Isaac Asimov: Enjoyable, though for me most later Asimov is nowhere near as captivating as other good SF. Avoid “collaborations” – other writers working from an Asimov story.
Larry Niven: Remarkable. Perhaps the only person to have paved as much new ground as Heinlein. Niven’s characterizations are weak, and their motivations sometimes hard to grasp, but his story lines carry the story along extremely well. Avoid Ringworld and, especially, its sequels until after having read other “Known Space” books.
Jerry Pournelle: Much of Jerry’s solo work is “military SF” and not terribly interesting, but his collaborations, especially with Niven (with or without Steven Barnes) and with Stirling, are remarkable. In fact, Niven and Pournelle as a team are head and shoulders above either writing separately.
Lois McMaster Bujold: Imagine Georgette Heyer, E.E. Smith, and Tom Clancy doing a three-way collaboration, with James Michener helping out on the family-saga aspects. The Vorkosigan saga is all that and more. She also has a fantasy series going, which I am not familiar with.
Marion Zimmer Bradley: MZB started out as a self-taught pulp author, and her works vary from slight to deep. But no one else has ever done so thorough a job of painting a culture in which psychic powers are dealt with on a daily basis. And she pulled this off over a highly-productive 40-year period.
Julian May: Baroque and Jungian, but man, can she tell a story!
Vernor Vinge: Whether space opera epic or short story, he can captivate. The Realtime and Zones of Thought series are especially good.
H. Beam Piper: His suicide back in the 60s leaves a mixed assortment of work. Both the Paratime/Kalvan series and the Fuzzy series are enjoyable reads; other work is variable but generally good.
John Barnes: It’s hard to explain why I like his writing, but, though horrified at some plot developments, I’ve almost always found a Barnes novel worth reading.
Spider Robinson: Heinlein on acid, with puns that make me groan. I’ve never read a Robinson I didn’t enjoy, though the last few Callahan’s stories left me feeling that he’s painted himself into a corner in that series and is now doing an enjoyable tap dance in the small space he left himself. (He’s also taken the Callahan crew off the Reichenberg Falls so many times they’ve opened up a barrel franchise.)
Poul Anderson: An acquired taste. I find his work nearly always enjoyable, but many do not. He has a thing for competent hypermasculine heroes who singlehandedly pull off the highly improbable against astronomic odds. Second only to Niven in setting up odd astronomic situations and making them work as story settings.