he's one more who would have been on my list if i'd taken more time to think about it. i've actually got masks of the martyrs kicking arround the house somewhere. =^^= .../\...
My two favourites are Wells and Clarke. Wells was kinda forerunner of all these sci-fi thematics that have become classic.
Robert A. Heinlein, Ursula K. Le Guin, Robert J. Sawyer, and Cory Doctorow. These authors have changed the way I see and approach my world.
Sci-fi is not one of my strong points, but these 3 will always have a special place: H. P. Lovecraft, Douglas Adams, Clive Barker. Lovecraft is among my top 5 of all time, but he's not strictly sci-fi, so...
John Varley is the best. His new trilogy RED THUNDER, RED LIGHTENING, & ROLLING THUNDER are awesome. I don't think it beat his first trilogy TITAN, WIZARD, & DEMON tho. One of his books MILLENNIUM was made into a movie with Sheril Ladd & Chris Christofferson.
John Wyndham by a country mile - He's fucking great. Also Mr Dick, though I prefer his non SF stuff really.