First of all, British is not an empire, or at least a classic empire. It's called the British "Empire" but this word is very off. Britain, when formed, was a monarchy, ruled by a king. In Colonial Age, Britain was considered a Nation-State, just like France and Prussian "Empire," although Prussia ended in Franco-Prussian war and Germany was formed, but the official Germany as we know of today did not exist till Hilter.
True empires are Roman, Greece, Mesopotamian, China, ect.
And as greatest, I would nominate Chinese Empire, the only classic empire existed to this day.