I would definitely go with the Mongols also.
They were nearly victorious in a total land conquest in Asia.
Their army organization, logistical advantages, tactics, and overall strategy make them the best army, for their time.
"With Heaven's aid I have conquered for you a huge empire. But my life was too short to achieve the conquest of the world. That task is left for you."
Which is why the Mongols truly failed. Once Genghis was gone, they had to fracture.
They were organized in groups of tens(arban), hundreds(jagun), thousands(minghaan), and ten thousands(Tumen).
Each of these groups elected from among themselves a leader, except in the case of a general, prince, or Khan taking command.
On the most basic level, every Arban had a tent, food supplies, and their equipment carried among themselves; most warriors had multiple horses. On long or forced/desparate marches, they would eat, sleep, and sometimes even relieve themselves from the saddle. When food was low, dried milk curds(softened by being mixed with water and hung under the saddle while travelling) & dried meat were staples. When those ran low, the troops could drink the milk of the mares, and even some of the horse's plentiful blood.
They could do this for weeks at a time, while keeping a pace that is faster than most artillery columns.
With such speed they could outmaneuver virtually every other army of the time.
Their very tactics were often all the advantage they needed. The Mongols were known to be the greatest horse archers in the world. Able to draw a 100 lb composite bow and shoot accurately while at full gallop, forwards or backwards. I find it believable, since they were usually trained from as soon as they were able in riding, archery,
and wrestling.
And finally, they were known to have some of the best generals of their time.
Ones such as Tsubodai, Jebe, Genghis himself, and many of their other Khans. Mongol military leaders almost always had their rank because of ability, not birth. Even a son of the Khan would not be granted command of any sort unless he was considered fit to have it.
Edit: I might also note they advanced beyond the borders of territory they knew existed as well.