If you want to build a balanced, more realistic army I would go with Vaegir or Sarranid. Crossbows are better than bows in siege because they don't run out of ammo as fast as bows and they still pack a punch. Their cavalry can take care of any field battle while their heavy infantry will hold their own in a siege. Swadia would probably be the best if it wasn't in the center of the map at war with everyone. Khergit has horse archers and cheap units so you can build an army fast. Rhodok has best crossbowmen, but bad infantry and no cav. I don't really like playing Nords because I rather have good ranged units or cav so I don't know much about their other units.
Nords have best infantry, aka best siege unit, but are lacking elsewhere. Swadia makes the best field army, but it is very expensive to upgrade to max and upkeep. Swadia has best heavy cav, aka best field unit, and decent infantry and crossbows. The have mediocre infantry and cav with the best archers. If you want to build a balanced army go with either Vaegir or Sarranid.
