For a classroom exercise, my daughter just asked me to define influence and to name the most influential person in world history. My definition of influence was, "the ability to change people's minds," and I chose John Locke, although I gave serious consideration to Adam Smith. My rationale is that Locke was a major influence on the American revolution and Constitution, which in turn were major historical events.
In retrospect, perhaps I should have chosen Karl Marx. But for a high school student, better to pick someone whose influence was for the good.