Interesting topic, but it needs to be a bit more focused. You could have multiple answers and they'd all be right depending on one's criteria.
Are we talking largest land mass conquered? If that's the case, it would be Khan, whose Mongol dynasty covered more land than even Rome in its height.
Greatest conqueror in terms of leadership ability and/or ingenuity? Then I'd go with Napoleon or Alexander, who both revolutionized the way people thought about warfare in their time, and were both highly revered.
Or simply influence and power? Then I'd have to go with some Roman Emperor at the height of Rome's power (probably not Julius). They were certainly the most culturally influential of the great empires of human history.
...there's probably some I'm forgetting, or that haven't been mentioned yet, but it's certainly an interesting discussion by any criteria. Cool thread.