Considering that the human population of the world has been increasing exponentially since whenever it can be considered to have begun, it stands to reason that the most influential person must have lived during a time of extremely low human population, near the emergence of the species from the pregenitor stock. The further back in time you go, the more impact each individual has, because there are fewer individuals.

Logically, it seems most likely that the most influential individual must have lived before the population explosion that occurred about fifty thousand years ago. The best chances are for someone in East Africa at least two hunded thousand years before the New Kingdom of Egypt. That is if you include humans only and not pre-human hominids.

If you include pre-human hominids, it quickly becomes considerably more complicated, as you begin to rely even more on genetic causality than on basic middle-world causality.