Who's my favorite author? It varies over time.
Really, it's a hard question. It may not seem like it, but it is. I've been reading science fiction and fantasy for more than forty years now, and I have a lot of favorites. If I have to name a single book I think is the best one out there, though, I guess I'd say Alfred Bester's The Stars My Destination. William Gibson's Neuromancer is excellent, too--and Robert Heinlein's The Door into Summer, and George R.R. Martin's Dying of the Light.
Then there are the authors who are consistently excellent without having one stand-out book, like Fritz Leiber and Terry Pratchett. (I Like Pratchett enough that I wrote an entire book about his work.)
But that's all staying within the genre. Outside, a favorite is C.S. Forester. He didn't write science fiction; he mostly wrote war stories. The work I enjoy the most, and which I read over and over, is the series he did about Horatio Hornblower, of the British Royal Navy--born 1776, served at sea from 1792 until 1823, rising from midshipman to admiral; there's also one short story set in 1848, long after he retired.
And I also admire Dorothy Sayers, Rex Stout, and Raymond Chandler, who wrote mysteries; Jane Austen and Emily Bronte, who wrote romances; Kurt Vonnegut and J.D. Salinger and various others. Getting back to SF and fantasy, Ray Bradbury and Anne McCaffrey and John Crowley should be mentioned.
Sorry if I'm giving too many names. There's so much good stuff out there that I hate to see people limit themselves to a single category, or just a few authors. I know there's plenty of junk, too, and you want to avoid wasting time on it.
That's it; here's your list of handy exits: