Who is, or who was, the most powerful person in history?
To answer this question, one has to define the term, “power.” Ask a physicist, and he or she will probably tell you something like, “power is the capacity to effect change on the physical world.” They may also point out that modern scientific convention says there are four fundamental forces which shaped the entire universe, being gravity, electricity, and something called the weak and strong electromagnetic forces. So, a physicist might answer this question by pointing to someone like Thomas Edison, commonly credited with the invention of the electric light bulb, or Albert Einstein, who split the atom and discovered atomic energy. No one has managed to harness the power of gravity as of yet, at least not in a way that has been patented and marketed to the general public. That said, you can read thousands of webpages about how the US government is hiding this technology in underground bunkers underneath top secret military bases like Area 51. That might be how a conspiracy theorist would answer the question of where to look for the most powerful person in history.
On the other hand, an academic or a scholar might bring up the old adage, “the pen is mightier than the sword,” and nominate candidates like Paul of Tarsus, who wrote most of the New Testament which was central in creating the largest religion in history, Christianity. Or they might point to Jesus of Nazareth himself, or possibly Sidhartha Gautama, more commonly known as the Buddha, or the prophet Mohammed, or the authors of the Upanishads, whose name has been lost to history. Who is to say that these respective traditions will someday surpass the popularity of Christianity. Or for that matter, an academic may well nominate Charles Darwin whose theory of evolution kicked off a groundswell of atheism. Maybe, someday, atheism will replace all other religions as the most popular faith, or lack of faith, as it were, in the world, and hence people like Darwin, or Richard Dawkins, or an as of yet unrecognized influential proponent of atheism could be the most powerful people in history.
Of course, there is another, arguably simpler, albeit grim way to judge who the most powerful person in history is. That would be whoever has the ability to unleash the most powerful agent of change hence conceived in all of history. At the present moment, the most powerful agent of change that I can think of would be the United States nuclear arsenal. At the moment, the person with the authority to unleash that arsenal upon the world is President Donald J. Trump. You may be surprised to learn that the president of the United States, whoever that may be, has the legal authority to order a nuclear strike without having to consult directly with anyone else. Now, there is a loophole to set at ease those who think that President Trump is an unpredictable blowhard who might unleash a nuclear winter as readily as he unleashes a blistering Twitter storm, which he does on a regular basis. The order to launch a nuclear attack must go through his military personnel. Obviously, it would not be Trump personally who pushes the “big red button,” as he himself has described it. And, it turns out that military personnel have a right to disobey any order which they believe to be illegal. They would have to defend their belief in court, and if they president has simply decided he really wanted to see what a mushroom cloud looks like, and so why not drop a nuke in the middle of the Sahara Desert, chances are the order would be defied and the president would be removed from office, much to the joy of his numerous critics. On the other hand, if a United States president decided to drop some nukes on, say, Pyonyang, Tehran, a dozen other targets in the Middle East and Northern Africa, maybe even half a dozen in China and Russia each… depending on circumstances, we could have a situation the media might call, Hiroshima times 100.
If that makes you uncomfortable, I advise you to count your blessings and remember that there was a time when the leaders of the most powerful states on earth had even less accountability than they have now. I am speaking of what historians call the absolute monarchs of medieval Europe, most notably, Philip II, Louis XIV, and Peter the Great.