Well if this is the middle Ages, then it would probably have to be the Golden Horde. Mostly for its organizational skills during battle, great idea using whistling arrows to signal different formations, as well as wearing silk under their armor to stop arrows from penetrating their body. Another great idea was the use of their pony express, which significantly improved their communacation. The kahns were certainly not barbarains, but a higly mobile and elite force. They were born riding a horse, and with a bow in their hand, and that's what they did best. I cannot say they had the best infantry, as their soldiers weren't higly trained for that, but they had the flexibility to crush almost any army. As for their bows, the horse riders would carry two bows, one for a long range, one for a shorter. I have to say that the longbow was ill in comparison to a mongolian composite bow. A mongolian bow could out range a longbow easily. The design of the composite bow was small for horse riding, and very thick, made out of layers of bone and sinew from animals. These people were riding horses and making bows before the english were hanging by their tales in the trees