Styles, themes and scientific knowledge have changed so much since the 'invention' of the genre, I have to break my list down chronologically (and it tends to short fiction - I grew up with the magazines and the "year's best" anthologies).

pre- 1950's: Asimov and Heinlein
Golden Age (1950's & 60s): Cordwainer Smith and Ray Bradbury
'post-Modern' SF (late 60's through 70s): Roger Zelazny, Samuel Delany and John Varley
Cyberpunk and Beyond: William Gibson and Michael Swanwick

Most consistent throught his career: Asimov
Most improved with age: Poul Anderson and Frederick Pohl
Farthest past prime while still writing: Heinlein and Clarke
Most entertaining in person: Alfred Bester
Best editor: Gardner Dozois

Personal favorite "soft" SF novel: Always Coming Home, Ursula K. LeGuin
Personal favorite "soft" SF short fiction: "The Persistence of Vision", John Varley
Personal favorite "hard" SF novel: Red Mars, Kim Stanley Robinson
Personal favorite "alternate history" novel: Jack Faust, Michael Swanwick
Personal favorite Heinlein novel: The Moon is a Harsh Mistress
Personal favorite Heinlein 'juvie': Between Planets

However, my shelf is also full of David Brin, and the list of authors of all time periods whose work I admire greatly (but don't own copies of) goes on for a loooooong time.