Ancient Civ with best cavalry

There is little argument needed. The armies of Murong Yan, Bei Wei, and Rouran are the most efficient. For the simple reason that they were among the first armies to use metal stirrups in mass. The Ephtalites defeated the Sassanians, and the Ephtalites were vassals of the Rouran, which in turn were defeated by the Wei under a series of campaigns. None of the other cavalry on the vote list come even close.

BTW, need I mention that the XianBei cavalry was superior to the Xiongnu, a straight quote taken out of Hou Han Shu:

"the Xianbei were able to acquire the developed iron weapons of the central plain so that their cavalry is faster and their weapons are sharper than those of the Xiongnu."

Even at this date, there was no full stirrups and the Yan army is the first army in history from archeological findings that deployed heavy double metal stirruped cavalry in mass with decisive results. Although the Han army of Liu Yuan(xiongnu Han) might very well have double stirrups already, its very clear from first hand documents that the later Zhao under the Shi family already had such cavalry.

People tend to vote for whatever is closest to their own nationality.