I became a science fiction fan when, as a young teen, I read my first Robert A. Heinlein book -- specifically, DOUBLE STAR. With experience (in reading sf) it has become harder to say, as I once did, that he was my favorite author.
Now, to me, there are many sf authors whose work I love and admire, and I find it impossible to list them as "best," or "next best," or whatever. So I'll just throw out the names of some great stories as they come to my mind now (with the understanding that I will likely, two hours from now, realize that I failed to mention this or that great one):
WAY STATION by Clifford D. Simak
HEAVY TIME by C.J. Cherryh
CREATURES OF LIGHT AND DARKNESS by Roger Zelazny
CITY AT THE END OF TIME by Edmond Hamilton
THE FOREVER WAR by Joe Haldeman
I, ROBOT by Isaac Asimov
DUNE by Frank Herbert
THE SHINING by Stephen King
Oh, and so, so many more. But that will give you a start.