Arthur C Clark
I have only read a few of his books (I read, and greatly enjoyed, 2001: A Space Odyssey and then something distracted me before I finished 2010: Odyssey Two) but he has had a huge influence on my life, if only as a byproduct of the influence he has had on the world as a whole.

He was among the first to think about and formally propose the concept of a geostationary satellite, a technology that is the basis of almost all global telecommunication systems, and he was always at the forefront of scientific knowledge. He also brought the concept of space elevators to his audience, an act he believed would leave a far greater legacy than even the satellites.

Really, there's nothing more I can say. He was a great man.