Who had the greatest of all? Single word or numerical answer.
I think the answer is
Robert Wadlow, who wore US size 37AA (UK 36) shoes. (For a single word answer, then I suppose just "Wadlow".)
When you multiply the numbers on the calculators together the product is $3215.3045$. Turn that upside-down on a calculator display like those in the picture and it reads SHOE.SIZE.
I think the answer is :
When read upside down on a calculator, this spells "Bezos", the man with the greatest fortune and the owner of Amazon (the Amazon river is pictured as far as I can tell). I believe the "Go Forth and..." quote is a red herring to imply multiplication. It could be read as "Go Forth and Prosper," which Bezos has done, no doubt. Although, I am suspicious that the "who had" question could imply a dead person...