What is the largest Ponzi scheme of all time?

Banks are.

Why?

—Both rely on a continuous flow of new participants (depositors or investors) to sustain their operations

—Both involve the management of third-party funds and investments.

If all investors withdraw the funds invested (bank run) the business collapses.

Just because banks are regulated does not mean they are not a Ponzi, in fact they are highly regulated for that reason.