(one is probably already in trouble by claiming 'the greatest of all time' but)
"Insanity is relative. It depends on who has who locked in what cage."
- Ray Bradbury
"..science fiction is something that could happen - but usually you wouldn't want it to. Fantasy is something that couldn't happen - though often you only wish that it could."
- Arthur C. Clarke
"If you think this Universe is bad, you should see some of the others."
- Philip K. Dick
"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects."
- Robert A. Heinlein (Time Enough for Love)
Many More:
http://www.outerplaces.com/science-f...iction-writers