First off, you need to state what you mean by ancient times. That's such a broad term that could mean anything. The 1500's could be considered ancient time.
Second, in what region? The Pope had/has no pull over in Asia. Ghengis Khan is one of the 10 most powerful people in history (not counting U.S. presidents), so him being an emperor, he would surely be your answer.
I would say his son Ogedey was the most powerful single person ever. The ruler of the largest contiguous empire in world history, and the leader of a military that was so advanced it would probably have beaten any army up until the invention of cars and planes more than 600 years later.