Rubbish! Augustus Caesar may have been the greatest Emperor, but he was no General (I'm not sure he ever personally commanded his troops). Julius Caesar, Scipio, Gaius Marius - you could make a case for any of them (although they're all from the Republican period, as I know less about the Imperial period of Roman history).
I once read a book on military history written by Field Marshall Montgomery, in which he tried to grade military commanders according to their achievements. He said that there were six who stood out above the rest not just because of the victories they had achieved but also because of the size of the armies they commanded. The six were : Hannibal, Napoleon, Frederick the Great, Gustavus Adolphus (Sweden), Genghis Khan and Julius Ceasar.
This is an unanswerable question because it is unknowable. Each man faced different opponents in different contexts. Some who won great battles made terrible foreign policy blunders and vice-versa.