Cyrus the Great
Alexander the Great
Alexander the Great is arguably the best military commander in history, with the only other contender (in my mind) being Genghis Khan. I'd personally lean towards Alexander. While it's true that Genghis Kahn's empire was bigger, Kahn conquered a mostly barren landscape nearly devoid of humans with the exceptions of a bit of eastern Europe and the far East. In order to do this, exceptions aside, he basically had to go somewhere with an army and say that it was his. There was no one there to challenge him. Also, it took Genghis Kahn decades to do this. Alexander the Great conquered an area about a third the size of Kahn's, which was at the time the most heavily populated area on the planet, fighting against the most powerful empire in the world up until that time, regularly engaging in battles in which he was outnumbered 10 to 1, and it only took him about a sixth as much time as it took Kahn. Mathematically speaking, if Alexander hadn't died young (32 or 33 if memory serves), he would have taken a greater area than Kahn. But I digress. A lot. I seem to have gone into a bit of a rant. I was just comparing Alexander to arguably the next-best conqueror to prove that he was the better of the two and therefore best of all. In a word, Alexander conquered the empire of Cyrus anyway, although it was from the hands of a later king.