"A Celestial Experience" (Level 88 quest on Pandaria)
This quest involves talking to somebody, and killing 3 bosses.
Each of the 3 bosses grants 40k exp, and is rather easy to kill.
You can abandon this quest and redo it, over and over, I've farmed a quarter of a level on this quest in half an hour, where that would've taken about 1 and a half hours according to HB.