The President of The United States is often called "the most powerful man in the world." Have there been times in history when that distinction could have reasonably applied to anyone else?

Yes, and with much greater force. The American president's powers are naturally diluted, in that the executive must share power not only with concurrent federal branches of government, the legislative and the judiciary, but also with state governments. Compare this to the leadership in a dictatorship or a more authoritarian state, or even a republic without so many checks and balances on executive power, and you will possibly find a more powerful individual. I would suggest that Vladimir Putin, for instance, though there are some checks against his power, wields more power than President Obama, because he does not have nearly as many restrictions on his power. Consider just how little President Obama has been able to guide policy in a direction he would like, with even his successful initiatives facing gridlock and blowback from other branches and the states. At the same time, if you are measuring power on the basis of military might at one's disposal, President Obama probably wins in any present contest.

In history, there are plenty examples of an individual with more power than the American president. Hitler, Napoleon, Stalin, and Mao are the best examples in the modern era. In the ancien régime, the monarchs of Europe held significant power, without democratic checks, and of course in the East we can look to the rulers of the Arab conquerors, the Ottomans, the Mongols, the Chinese, and the Japanese. Ancient history, of course, is rich with other examples.

A lot of this might depend on how you weigh power. If your judgment is "power over individuals" and not power based on technological advancements (like the ability to destroy a metropolitan area in seconds with a hydrogen bomb), then you have a lot more wiggle room. Otherwise, you are restricted to recent history.