Without doubt Genghis Khan.
Nomadic Mongol clans were in a constant state of strife with one another, and were positioned right next door to the greatest human civilisation at the time China. Temojin (Genghis Khan) miraculously united all the clans through various alliances and managed to conquer China and half of the known world with it.
This is so great an achievement that the modern equivalent would be if Nicaragua, Honduras, El Salvador and Guatemala banded together, and went on a rampage conquering Mexico then the US and Canada, no other military leader comes close to Genghis Khan.
In my opinion what further cements him as the greatest military leader is that he created a lineage of equally accomplished military leaders that went on to do incredible things. Genghis Khan's grandson for instance is the great Hulagu Khan, the man who pacified most of western Asia.