All Warlock classes are awesome with the right setup. Voidwalkers with Obsidian Mind and correctly configured for max Energy Drain and shit can straight up keep spamming Nova Bombs all day. I highly doubt any other sub-class in the game can recharge their super as fast as an Obsidian Mind Voidwalker.
Sunsingers are great with max Discipline and Radiant Skin because you're basically an orb-generating machine with your super. I've stopped using Heart of Praxic Fire because Bungie nerfed its weapon-specific perks, but with max Discipline and Heart of Praxic Fire, A Radiant Skin Sunsinger is a monster in both PVE and PVP. A word of advice, though, relying on Fireborn as a Sunsinger can be really crippling to your playstyle, especially in Year 2, where being aggressive is rewarded and turtling is a death sentence in most cases. The only place you should be using Fireborn is during the final Oryx encounter in Heroic King's Fall.
As for Stormcallers, they don't really have any "must-have" exotics (yet), but simply using Transcendence perk makes them the best Add-clearing class in the game alongside Sunbreakers. The added bonus of Health regeneration means that you can be ultra-aggressive and just lay into mobs with swords and when you're about to die, just activate your super and go ham on them again.