As suggested by Casura:
For me, it'd be mission rooms, where I climactic crash, first hit crit so everything's dead, then second hit crit so everything's dead again, and I crash shot (or wm) crit so that I one shot the next spawn and the room is done. Such easiness. Much win.
Oh, and also, when mobs get stuck in Sulfur Spider. Or Scathach mission.