I  think  Sacred  Order  has  the  most  amount  of  useful  champions  all  around  (not  to  mention  having  the  best  uncommon  in  the  game).

If I understand your question right (I'm not sure about that), you want to know wich faction has the best "arithmetic mean", if you add the rating of all champions. Your example is misleading, as Bandit (Bannerlords) and Big 'Un (Ogryn) are in different factions.

I guess Shadowkin would win that fight, they are a small faction, without commens and a great ratio of legendaries compared to the overall number of champions. That's giving them a very good average.

thanks! fixed it.