Well I think we can agree that Megas alexandros was one of the (if not THE) greatest generals in Antiquity. (if we take the period of Antiquity to last until the birth of Christ.) Who could be the greatest general of the "dark ages" (if we take it from the birt of christ till the fall of the western Roman empire) I couldn't say (although I think Great King Attila would surely be a potential candidate). The greatest General of the Middle ages would probeably be Genghis Khan but I can"t be sure.

But now back to topic: I believe the Nomad factions might have the most easiest to get good generals - since they often need there Bodyguard Horsemen to decide the battles and they can easely win battles where the chances are seriously against them.