Karl Marx. He's challenging, but no man in history has ever put together a theory of human behavior and economic/social development more powerful or more accurate than he has.
He's regularly rated as one of the most influential men in history, certainly he has made a massive impact on our current level of social and economic development.
Despite what people think, his influence didn't start or end with the USSR. His theories and ideas are carried in nearly every nation, both Democrats and Republicans would find themselves stripped bare without his demands; Europe would be amazed to see how closely they are following his prescriptions. Whole nations owe their existence to his ideas, whole people owe their liberation to him, billions owe their lives to him, and millions can blame him for their death.
Noone in history has laid out economic and historical tendencies perfectly, so as to actually give us a view into a future not yet seen.
Few men or women have written books that can still be read hundreds of years after their author has died; read with the same wonder and insight as the day they were published.
He's a hard read, but god is it a satisfying one.
Check out Capital, The Communist Manifesto, and amalgamated works of both him and Engels.