Pretty sure Britain colonised roughly a third of the world under a unified leader, hence "Great" Britain.. Also I'm rather certain that the German empire also "unified" all the different Prussian States.. Lastly American isn't an empire and it never has had an empire, you simply colonised a land mass again an inferior type of people which isn't much to be proud of.. Personally I think Britain had one of the most powerful empires (Not only because I'm English) But because a third of the worlds land mass is a huge accomplishment, under the rule of a woman too who were degraded at the time :)