What is best merc for caster in 1.10??

I pretty much only use might mercs. Only time I don't is with my blizz sorc (defiance). In reply to the other post: I don't know about Doom for a necro that summons (i.e. skeles, resurect) With the cold damage you wouldn't have as much bodys lying around. You would need them for either your army, or for CE. Doom works well on other charaters' mercs, like a Strafe/Mulit Zon or any Melee build.

You could always go A2 Thorns Merc for a summoner...