Charles Darwin.

He has had more effect on the way educated people from every country and background view life on earth than any man I can think of. And he wrote for the layman so that his ideas would be clear, concise, and understood. He layed out his ideas and evidence like a well written law brief. He also pointed out all the weak points in his ideas (which have now been explained). A very humble intellectual that wanted nothing more than to help people see the truth.
No evil intentions as some Neaderthals would have one believe, just an incredible view of all life and the mechanism by which populations change.

He is the MAN.