The three largest empires in west Africa in 400 BC were the Askumite Empire. Carthage, and the Egyptian empire. All flourished well.

It is difficult to answer. are you asking the biggest in size or in population and if in population it depends on the relative world population at that time. Any how, refer to links belowfor more information.

The largest and most powerful Muslim empire was the Ottoman Empire.

These have been the largest in order of land area controlled.

British. Mongol. Russian. Spanish