In D1 it seemed to be Voidwalker. I'm not saying this changed in D2, I don't know, but I've been seeing a lot of Stormcallers lately. When I play public events and a Warlock activates the super, it's more often Stormtrance than Nova Bomb or Dawnbreak.
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Most popular in D1 was sunsinger without question. In D2 I don't know but I can say stormcaller is the most effective due to arc soul.