Many people would answer this question with a simple word: GOD. But significantly, nowhere does the bible say that either Jesus Christ or his Father are the real rulers of this world. On the contrary, Jesus said:"The ruler of this world will be cast out" And he added:"The ruler of this world is coming. And he has no hold on me."- John 12:31; 14:30; 16:11.
So the ruler of this world is in opposition to Jesus. Who do you think this could be?