I would have to say Ghengis Khan.
It was not until 1206 that Genghis was named Khan of Khans or King of Kings and king of ‘all people who lived in felt tents’. With all of the Mongol tribes united and under his control he could now concentrate his forces on expanding his empire.
In 1207 he began a crusade to conquer the lands of China. At that time China was divided into three separate empires. They were the Qin, Tangut empires in the north and the Sung Empire in the South. He himself led battles against the Tangut state in what is now present day Xinjiang (northwest China), and the Qin in northern China, taking Peking in 1215. However, although most of northern China was under Mongol control Genghis's dream to dominate all Chinese territory would be achieved but occur until the reign his grandson Kublai Khan in 1279.
With northern China under his control he now turned his attention westward. In 1218, the Khwarazm (modern Uzbekistan) Shah, Mohammed II, slaughtered a Mongolian caravan and a following delegation of ambassadors. This precipitated Chinghis's attacks on Central Asia, although in any case it may well have been merely a matter of time before he attacked. Genghis sent a message to their leader Shah Mohammed, saying that the governor must be turned over to the Mongols or war would be declared on Kwarezm.
The Kwarezm Empire refused and war was declared. Genghis led an attack force of 90,000 men from the north and he sent a general with 30,000 men to attack from the east. Despite this large army he was outnumbered by the Shah's army more than 400,000 men. Genghis's army was victorious, allowing a full scale invasion and occupation of the Kwarezm Empire. From this campaign the Mongols acquired the knowledge of the "fire that flies", burning arrows. And with subsequent victories new methods of warfare were used to made his armies stronger and more deadly.