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Quickest leveling (pre-roleplay).
I leave the Keep, accept potion of quickness from, Imoen, remove her from party, travel to the farmstead north, drink potion, enter the caves, grab the loot in the cave avoiding interaction with enemies, travel to Durlag's tower, stun the Death Knights with the wand from the loot, slay them, and earn 4000 XP.
Quickest way I have found so far.
Any other takes?