Morosa for the reason mentioned in the post below. The chain lightning would always seem to get themselves into far to much trouble with other mobs in the room, for the amount of damage its chain lightning often did.

The draw back for Mors Gravis makes it next to useless unless you are gaunting goodies. For ranking, if not killing evil or neutral mobs, it gets itself killed quick by lashing out all goodies nearby(which though, when used properly can be devestating).