I would say the Assyrian Empire at their height where tougher when Sennecherib ruled. Heads on sticks outside the walled city, Public flaying and boiling of people, and Military schooling at age 9 where the norm, had control over northern Africa, southern Europe, the entire middle east and parts of Asia. The Babylonians, the Persians and Greeks were tough too. But far more impressive where the Romans then the Mongals 4 centuries of ruling over 25% of the earth population and not only that but controlled about 60% of all the cities metropolis in the world, a feat that no race can claim. Ya the Mongals ruled more in terms of fractions, like maybe 40% of the worlds population, but only for a short period (2 generations), and mostly extremely populated tribal areas(certainly not 40%of the civilized world) and then crumbelled, anyone knows you can't govern over that large of an area without being overthrown or resulting in civil war. Just think had the Romans concentrated their forces and conquested in all direction without thinking of the repercusions that would befall 20-30-40 years down the road, they would have ruled over half the earth if not more, but clearly not a good idea if your concerned over the future stability of the empire.
So I would rank the Romans as number 1, the Assyrians number 2, America number 3 in terms of powerful Empires and maybe Mongals 4th or 5th
