I can't say the presidency of our fair land is the hardest job in the world, there is no way to know. I can say it is a very hard job.
I remember with some clarity all eleven presidents going back to Kennedy. I was aware of Truman and Eisenhower but, as a young person, didn't pay a whole lot of attention.
Every president in my memory has aged dramatically in office, especially the two term presidents. None of them came out looking as worn out as Biden will going in, if elected. The office takes a lot out of those who win it, and I'm not certain Joe Biden has much left to give.
Say what you will about Donald Trump, he gives every indication of surviving another 4 years of Democratic abuse and may even have a good time in the process. He'll get my vote.