I always go for Battanian Archers bcs they are the best but imo there need to be something like a "Moral" system so that when you have 2 mixed cultures they only fight 90% effective in battle and if u have 3 then its 80%.
Otherwise it makes no sense.
Or another idea is giving every faction bonus and penalties on different battle scenes, like Battanian 10% more accuracy in Wood/Forest, Aserai faster or more dmg in the desert, sturgia 10% movement in Snow (1.5.7) , khuzait only in their country 10% more movement and so on