NPC: Final Doubt, Lingering Doubt (330k HP, 3k melee hit)
Drop chance: 10% (Based on 1h)
Amount of NPCs available: 30-40 if nobody else is farming, instant respawn.
Background: Just found this area while doing the August Celestials' daily quests. As a protection warrior I can pull NPC packs of 20-40, AoE them down and loot 69 motes of harmony in less than 30 minutes of time.