The Roman Emperor Augustus was believed to have been worth in current dollar terms nearly $5 trillion.
The only other person to have reached the trillion dollars net worth status was King Solomon of Judaea. After octavian/Augustus defeated Marc Antony and Cleopatra, he then possessed the entire wealth of Egypt. In this respect, the wealth really did belong to him. The Roman system was that a governor paid the tax to the state and he had the right to mine that province for taxes which were always in excess. So in this perspective, the wealth of Egypt truly did belong to Octavian/Augustus.
Some have attributed the entire wealth of nations conquered and argued that Genghis Khan was worth probably $100 trillion dollars. However, Genghis Khan may have been the ruler, but this was not exactly the same as the Roman system. All taxation was not the personal property of Genghis Khan as was the case for a governor of a province in the Roman Empire.