My favorite SF authors are (in order of when they were my favorite):

Isaac Asimov was undoubtedly one of the most prolific writers of all time.  He wrote over 200 books covering many fields including science fiction, popular science, and religion. He wrote "I, Robot", a collection of short stories based upon his Three Laws of Robotics and the basis for the movie of the same name.  His Foundation trilogy was perhaps his most ambitious project. Many people know of Isaac Asimov not for his science fiction, but rather for his scores of books popularizing science.
	Arthur C Clarke may be best known for "2001 A Space Odyssey", yet he is a very prolific writer whose themes range widely.  Many of his books deal with human contact with highly advanced intelligences.  Many of the ideas behind his stories have since become reality or are seriously considered as future technologies.  One of "Clarke's Laws" is quite famous: Any sufficiently advanced technology is indistinguishable from magic.
Larry Niven is possibly the writer I most admire: his visions are many and varied, his stories engaging and interesting, his characters full and believable, and perhaps his most unique skill is that his aliens are alien. Most SF writers have aliens that act like people in funny costumes, but not Niven, who often bases a story on the fundamental differences between us and them. He is famous for his Known Space universe which apparently concludes with the Ringworld series. Another of his novels, Lucifer's Hammer, has been praised as the ultimate story about the end of the world from a meteor strike.

A few of my favorite SF books follow - an exhaustive list would be boringly long:

	Across Realtime by Vernor Vinge introduced the concept of the technological singularity - a point in the near future where the pace of innovation/computing/technology goes exponential resulting in an unknowable future.  Dozens of web sites, scientific papers, even conferences have resulted.		A Fire Upon The Deep by Vernor Vinge explores the implications of a universe where intelligence, computing and faster than light travel are all limited by the proximity to the galactic core.
	Neutron Star by Larry Niven is a collection of short stories set in the Known Space series, and explores a number of hard SF concepts ranging from neutron star tides to an exploding Milky Way Galaxy to a planet made entirely of antimatter.		Ringworld / Ringworld Engineers / Ringworld Throne / Ringworld's Children by Larry Niven are novels about an incredible artifact - a ring around a sun a million miles wide and 200 million in diameter.
	Rendezvous With Rama by Arthur C. Clarke is about a group of people that investigate an apparently dead miles-long spaceship hurtling through the solar system.  The imagery is vivid and immense. There are a number of sequels to answer old questions and ask new ones.		Gateway by Frederick Pohl is the first of the Heechee series, in which humanity explores the galaxy using found technology from an advanced yet extinct alien intelligence.
	Lucifer's Hammer by Larry Niven and Jerry Pournnelle is a detailed, multi-viewpoint story about a 4 mile wide comet that strikes the earth.  It goes beyond "setting the standard" as it sets the bar so high that it will be a long time before another author tackles this topic convincingly.		The Mote In God's Eye / The Gripping Hand by Larry Niven and Jerry Pournelle explores first contact with a superior and aggressive alien intelligence (where even the pets are tool makers), and how we can survive the encounter.
	I, Robot by Isaac Asimov is a series of short stories that each explore the implications of Asimov's "Three Laws of Robotics" (introduced here). There are several other collections and novels based upon the "I, Robot" universe, all of which paint robots in a positive light.		Berserkers by Fred Saberhagan takes a darker view of robots, presenting a universe where a doomsday device of self replicating machines endeavors to destroy all life. This collection of short stories spawned a dozen more, often by other authors.