It is an interesting question because it brings up the issue of potential v. actual power. Trump should easily be the most powerful person in the world because the US is still the largest economy in the world and has the largest military. But Trump is politically boxed in by the stalemate between the left and right in congress. Trump also is inept and has only been able to accomplish things through inaction by having agencies pull back from their regulatory duties. And militarily, the US has been out maneuvered by Russia in Syria and China in North Korea. That is not to say that Russia and China are walking all over the US. It is just that the US role is now basically to keep Russia and China in check.
Xi and Putin look powerful because of how they rule. But Russia's economy is tiny compared to the US. Putin rules at the pleasure of various oligarchs and has significant and growing opposition at home. China's economy walks a tenuous tight rope of overproduction bubbles and unmet consumer demand and widespread extreme poverty. China is closer to breaking apart than many realize with ongoing strife between Muslim Uyghurs, Tibetan Buddhists, Hong Kong, Taiwan and the nation government. Plus the ever present democracy and liberalization movement in China that stands in direct opposition to Xi's power grabs.
I will go with Mohammed Bin Salman. MBS has more actual power right now because he pretty much has free reign over the Saudi government. The Saudi military is comparably weak, but they are doing as they please in Yemen against the Houthi rebels. The Saudis got the US to back their move against Qatar with little question. MBS basically runs OPEC and could again tank the US shale industry at any moment. Conversely, MBS could tank the world economy by significantly pulling back supplies (Saudis are just about pumping full out right now).