Summons are not about damage, more about tanking. And in that case elementals are the best, as they can totaly negate some type of damage. When fighting fire enemies, a fire elemental can't harm them, but they can't harm it back.
And when the elementals do take damage, you can heal them while also harming enemies (pop a summon, let enemies cluster around it and toss some same element aoe at them)
If you want damage, from my experience the witchcraft skeletons are the strongest.
