If you put them all on equal technological footing.....the Romans would win. If not...the Ottomans. The Romans essentially dominated the known world in some form or another for over a thousand years. People think of the Roman Empire falling in 476 AD, but that was only the collapse of the [i]western [/i] Empire. The EASTERN Roman (Byzantine) Empire carried on for another thousand years, until it finally weakened to the point where the Ottomans were finally able to conquer it. The Romans dominated the world for that long because they were the worlds first PROFESSIONAL army. All the elements that we think off when we look at a modern military force were there in the Roman military. Training. Drilling and "war gaming" as organized units. Formal Discipline. Organized command-and-control. Combat engineering. Weapons R&D. (The [i]gladius[/i] was the---IIRC---the worlds first steel sword that was produced in large quantity, and its size and shape was ideally suited for Roman military tactics.) This professionalization of military service and war-making, allowed them to subdue their rivals, and to conquer "barbarian" forces that were much larger in size.....but where not organized enough to leverage their superior numbers. The Persian Empire dominated through sheer size. They were just able to put enough men on the battlefield that they overwhelmed their opponents. (The Greeks fended off the Persians twice...but got rolled by the Romans. The flexibility of legionary formations and tactics actually turning the Phalanx into a liability rather than an advantage). The Mongols relied on speed and terror tactics. Since they all fought from horseback, and and against other loosely organized forces, they were able to overwhelm their opposition. Plus the absolute brutality of their tactics (Genghis Khan was almost certainly a psychopath) also tended to suppress opposition. Not until the Ottomans did the Romans really face an opponent that was truly their equal.