Really I have to with the Roman Empire/Greek Civilization followed by the Spanish Empire. It's really hard to judge the American superpower right now considering it hasn't fallen yet or ended its reign. Can't judge the scope of its influence until we've seen it lose that influence (and it eventually will but who knows when).
So until we are able to judge that going with the Romans who had Greek influence of course. It's hard to find anything in western civilization that doesn't have some roots back to the Romans. Which is why I can't really give it to the British or Spaniards who have their influence from the Romans/Greeks. Truly the culture the Greeks and Romans crafted was one of the most influential in the world even til this day. And I think a lot of that just has to do with so many people in the west being fascinated with the Romans and Greeks. They were the epitome of what western civilization wanted to be and what it still strives to be (which is rather funny when you think about it but it is the perception).
