since the senate was appointing counsels up through the day Julius brought his army into Rome, I will say him for a very short period. Then of course Augustus.
There is a difference between having an empire and having an emperor as I explained before.
Where are you on the US Fall of Rome speculation?
If there is any parallel between the U.S. and Rome then I would say we're almost at the Marius /Sulla stage. Just to be a contrarian, I'll say the American Empire has yet to begin.