The Answer is Darius. (or Darius the First) Cyrus came first as the ruler of Persia that conquered a lot of territory. He then died, (from defending his land from Nomadic Invaders, of all things.) and passed the conquering to Darius, who was very aggressive and expanded the Kingdoms territory very wide. Up to the first cataract of the Nile. When Darius died, he then passed the reigns to Xerxes. (Xerxes just inherited already conquered land, and so he doesn't get credit for the work it took to get that land.) Xerxes did do some squelching of uprisings that were going on amongst citizens on his inherited land, so he was capable of some duties.