I think a "great" leader is one who makes a positive impact on people's lives. A great leader might help increase trade or encourage artists to create masterpieces. A great leader might also give people more freedom or power in the government.
Conquering the mighty Persian empire and others and thus creating one of the largest empires ever. He was King of all Greek nations, Shah of Persia, Pharaoh of Egypt and ruler of many more lands.
