I agree, F L E S H.. I would have to say Einstein, also.

My second choice would be Jesus. Whether he was real or not, or the son of God or not, the legend of Jesus has affected almost everyone and everything on this planet. If he had never existed, or - if the case is that he didnt exist - he had never been 'made up' then the world we currently live in would be completely different. The same goes for God. (PS, i'm not saying that either of them exist or existed, or that they didnt.. just that the 'idea' of them exists, and this 'idea' has affected the world more than anyone in history)