If you are talking comparatively, then yes it was easily Rome. At Rome's height they ruled most of the western world and were only seriously constrained by the logistics of the day in expanding further.
If you are talking about in absolute measure, then the US - though I would say that the US is certainly no where near Rome in comparative terms.
No Persia had the largest army in world history numbering in the millions, but Sparta had the strongest military in history they destroyed the much larger Persia force, adapting quickly to the Persian way of fighting, and quickly winning the war.
I would argue Genghis Khan and the Mongols.
Their empire stretched from China all the way to Vienna, and they stood on the brink of destroying all of Europe in the 1240s.
But for the good fortune of the death of the leader, the Mongols could well have ended western civilization in the space of a decade. They were notorious for burning entire cities to the ground and massacres of hundreds of thousands of people.
