Barnum had an influence, but certainly far more important influences on society were left off the list. Hefner's publishing titillating pictures fueled the entire sexual revolution of the 50s, 60s and beyond. He changed the sexual attitudes of women and altered relationships from predominantly marriage to predominantly shacking up. Along with the sexual freedoms and altered states of morality came more divorces and less religion in society. For these things I would put Hefner in the top 5 most influential people of America. He had almost as much influence on shaping America as did Washington, Jefferson, Lincoln, and FDR, and probably more so than Reagan.
Hefner is ineligible -- he is breathing!