I don't even know if the word "Empire" applies. I was told by someone a very long time ago that the "Nubian Empire", at it's height, was larger and more advanced than the Roman empire. They said that Plato's ideas about democracy were in part formed after several visits to the area. Is any of this true?
I'm not to sure about the Nubian Empire, but later on the the Egyptian Dynasties, there were Nubian Pharaohs. If you feel that the actual admission in Egyptian writings (they were rather biased against non-egyptions, ESPECIALLY Nubians in earlier dynasties) that a Pharaoh wasn't native born Egyptian, then you could say that the Nubian Empire was at one time at least as powerful as the Egyptian Empire.