Kurt Vonnegut - Dropped the Jnr. as at 80+ years he's gotta outlived his dear old papa. Brought a touch of humanity to SF writing and always amusing.
Robert Louis Stevenson - "Them that dies'll be the lucky ones"
George Orwell - Big Brother IS watching you!
Ray Bradbury - More than just science fiction.
China Miéville - The best current SF author. King Rat, Perdido Street Station, The Scar, Iron Council & now Looking for Jake (just out: Sept. 2005).
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Close misses
Philip K. Dick - Overlooked during his lifetime and died before Blade Runner was released as a film. Since then Total Recall, Screamers, Paycheck Minority Report have all been adapted.
Mervyn Peake - Gormenghast Trilogy is superb, was dissapointed by the BBC adaption which treated it as a Austen/Bronte soap opera. Would've included him if he'd produced one other great novel.
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Most dreaded author - Gotta be J.K. Rowling. The first two Potter novels were OK but since then they've grown alarmingly large. At over 700 pages Book 5 was a real drag, Book 6 at 600 pages not much better. However the sprog likes 'em.