Quest With Infinite Respawn Mob That Drops Loot (Patreon)

The quest "Rubble Rummaging" can be received by completing its pre-requisite "A Rousing Aroma" in north-western Revendreth. This seems to be only available during "Threads Of Fate" leveling. 

On this quest you are meant to search through piles of rubble for a key. Mostly you do not find the key and instead have an "ember worm" mob attack you. You can click on a pile of rubble multiple times and create infinite ember worms. Turn auto-loot off to do this as if you loot the actual rubble pile it will stop spawning mobs. 

The worm mobs award full experience. (This is unusual, most player-spawned mobs do not). 

The worms drop trash items, phantasmal haunches and shadowy shanks. The mobs can be skinned also. They also drop bind on equip items if you farm them long enough. Again this is unusual for any mob which can be spawned by player actions. 

Players with good gear and/or aoe abilities can farm the worms very fast, especially in a team. The amount of experience a team can get should be significantly greater than any Blizzard-approved leveling approach, for skilled players. 

There is a very similar exploit nearby, the treasure item "smuggled cache" spawns a "blistered hoarder" every time it is opened. You can open the cache endlessly and spawn infinite hoarders. The blistered hoarders do not drop loot, however they award rather more experience than the ember worms, but do more damage and are harder to kill. They can also only be spawned one at a time unlike the ember worms. 

EDIT: no longer awards xp.