I've found the Nords to be the most frustrating. They have a seemingly endless amount of troops, and said troops have a lot of variety when it comes to combat (huscarls w/ throwing axes, sword/shield, two handed axes). Although they lack cavalry, if they bunch up, they kill the horses and then swarm the rider.
Thats because the AI doesnt know how to ride a horse and just runs straight into the massive clumps rather than circling them/ picking off the stragglers
Edited:
As i said on this page I play Europe 1200