As of now, even the richest men in the world are subject to a country's law. So that would rule out for me the Bill Gates and Bezos of the world. Yes they wield a huge amount of power, but one law passed could really fukk their money and influence up. They do have power in the decisions they make, but even if they had a sayin in who will become president, the rest of the economic interests a country like the US has, would far outweight theirs.
The most powerful organization in the world is the western aliance led by the US, which would make the US president the most powerful man in the world, and in theory he is. But the US is a democracy and checks and balances really diminish this power by a lot, since most of his important decisions have to be approved by the congress or/and the supreme court.
And since there are other powerful countries in the world with a more authoritarian system, this would make their leaders a challenger to the US president as the most powerful man on earth. Of course the countries we are talking about are Russia and China.
China right now has a president who is trying to take all the power, but he still has to respond to the comunist party, in which there are other powerful figures other than Xi jinping. Even if it seems as he is the one who makes the most important decisions he still has to check in with the comunist party for this to happen.
This would leave Russia's Vladimir Putin for me as the most powerful man in the world right now. Russia is no longer an economic world power. Their direct influence doesnt extend beyond east europe and central asia. But they still have the second best military and nuclear power in the world. They provide natural gas to heat a large portion of western europe. They have vast natural resources and their location is geopolitical strategic point for both europe and asia.
And of course, Putin is the Czar of this Russia, he does as he wants and he doesnt have checks and balances.
So is Putin more powerful than the US and its western allies or China? Of course not. But to answer OPs question, as a singles man, Id say hes the most powerful man in the world.