I'm going to have to say Rome. Though they weren't the largest, nor the most powerful in History (That would be impossible seeing as in technology gets better which allows for better sailing IE Britain) I say they are the greatest because they are by far the most influential. Their legal system was highly advanced and was used, and improved upon by the British. Their language is responsible for all the major languages on earth (Except those in the east.) So in conclusion with that note. they may not have been as strong as Great Britain because the time would not allow it, but they influenced Great Britain greatly. Second I would like to bring up that when Rome was brought down is really debatable. When Constantinople became the second capital of Rome it drew a line in the Empire cutting it in half. Two sides of the same super power. When the western half broke through civil war and barbarian invasion the eastern half lived on. Through a name change they called themselves the Byzantine empire. Which, in my opinion, is no different then the roman empire it spanned from. Just less powerful.