I would have to concur that from a lore standpoint, Dhuum would probably be the most powerful, since even in a full party of 8 people, and the help of all the reapers, all that can be done is putting him back to sleep. Plus, correct me if I'm wrong, but the quest dialogues say nothing about how long Dhuum would be put back to sleep after we confront him. I guess at this point we could merge the lore and mechanics of the game to say that every time we enter the Underworld, Dhuum awakens once again, and therefore there is a constant struggle just to keep him dormant, let alone actually kill him.
It also seems that from the lore, the Great Destroyer could also be viewed as being extremely powerful, supposedly being hand-picked by Primordus, perhaps soon to be one of the most powerful creatures ever, as his/her general. If Primordus is as powerful as the lore suggests, he or she could have picked Shiro, the Lich Lord, possibly even Dhuum or Abaddon as the right-hand man. Remember, the power of the elder dragons is said to rival that of the gods, so if they really are that strong, recruiting a fallen god could be feasible, especially since it could help said god to regain lost power. The fact that Primordus selected the Great Destroyer says something about the power it has. Also, as with Dhuum, it could be possible that the Great Destroyer has recently come out of a dormant state, and therefore may not be using the full potential of his/her power at the time we actually confront him/her in game.