If you are roleplaying a "good" Merrill who does not use Blood Magic, then ironically her rivalry bonus is the best. Being her friend only benefits her BM abilities which you do not utilize at all if you don't follow the "Blood of the Fist" tree. I used Merril as Entropy/Spirit/Nature person and she still kicked ass. This is due to her bonus damage resistance, the upgraded armor she gets if you romance her, and the true awesomeness of the "Walking Bomb" & Spirit Bolt spells.

Isabella is the least useful as a companion. She dies way to easily and there is also a very serious bug related to her Friendship bonus.