Money magazine did a historical top ten list a couple of years ago based on the work of economists and historians and made estimates in 2014 dollars. John D. Rockefeller was #7 on the list at an estimated $341 billion, Bill Gates was #9, and Rockefeller's contemporary, Andrew Carnegie was #6. #2 was Caesar Augustus at $1.4 trillion (including personal ownership of Egypt). #1 was someone unknown to most people: Mansa Musa, the ruler of Mali around 1300 A.D. At the time, Mali was the largest producer of gold, by far, and as absolute ruler, Mansa Musa personally owned it all. No dollar estimate was given because it was so high as to be incomprehensible. In their words, "Richer than anyone could describe."
As pepsi_junkie points out, stock values are not liquid: when they start to sell large volumes the price will drop.
Happened to Mansu Mansa when he starting spreading his gold around Egypt and the MidEast: rampant inflation resulted, and his gold lost its value.