Robert Heinlein Isaac Asimov David Gerrold Robert Heinlein (i like him so much it merits two spots on the list.)
Alastair Reynolds. Hardcore sci-fi without alienating the reader too much. If you can suspend a little disbelief, he'll draw you in. As for soft sf It'd be Peter David.
Robert Heinlein, simply put he's a perfect author, his style is great. Friday was just what I was looking for!
My favourites in terms of quality would have to be: Ian M. Banks A.C. Clarke A. E. van Vogt I like Heinlein's stuff, but I find it can get pretty repetitive in terms of his ideas. And I find his style sometimes a struggle to read for any length of time. And then some authors have classics: Herbert's "Dune" for example, without me thinking them consistently good. But I'm also a fan of more pulpy series - like Bunch and Cole's Sten series, and Feintuch's Hope series. I find them easier to read and enjoy the basic ideas in them, but they're not literary masterpieces. Sort of like comparing a good action flick to Shakespeare.
My favorite author (of any genre but he writes SF and fantasy) is Alan Dean Foster. He's prolific, must write four new books every year. He has several series running; the human-thranx Commonwealth goes back 35 years but he also tosses out stand-alones periodically. He has quite a sense of humor and most of his stories have a few laughs--but not all. The Spellsinger series was a light-hearted fantasy about a UCLA student who is zapped into an alternate universe by a wizard who mis-cast a spell and it turns out that in that universe he has magical powers. Midworld is his shining achievement, a serious story about a human colony lost on an alien planet for so long that they mutate and swap a little DNA with the local ecosystem. My wife, the literature major who normally reads Shakespeare and Thomas Mann, thought this was a damn good read. I know that "serious" fans of speculative fiction sniff at Foster's work because SF is not supposed to be that kind of fun. But I'm always surprised that my two favorite "serious" sci fi authors never make your lists: James P. Hogan and the late Robert L. Forward, real scientists who create very interesting scientifically credible scenarios. You should check these guys out. As for fantasy, give Robin McKinley a read. Her books are generally marketed to young adults, but so is rock and roll and I love that too.