Arnold Kling asks (see here) who is the most influential person in world history, and suggests names like Locke, Smith, and Marx.

In my opinion this general question is biased towards the knuckleheads like Marx who were fabulously influential and wrong. This is because influential mistakes create something neither anticipated nor inevitable, while right ideas are somewhat inevitable. Thus good ideas are not so dependent on "great men" because there are lots of smart people and they eventually find the truth (witness the simultaneous discovery of things like evolution by Wallace and Darwin, calculus by Newton and Leibniz, or marginal analysis in economics by Menger, Jevons, and Walras). Bad ideas, in contrast, are infinite in number, and require a special magnetism and impenetrable self-assurance by their champions in order to become influential. Freud is a perfect example, a charlatan who befuddled two generations via his implacable self-esteem. Marx was similar, and Ayn Rand was cut from the same cloth but fortunately her radical ideas against empiricism never had as deleteriously wide an impact as Marx or Freud.

So for an individual to have great impact, it is probably in some wrong-headed idea about something not obviously falsifiable.