I don't quite know which skills do the most damage (preferably AoE) but have a relatively low spirit cost.
Here is the dps (damage per spirit) of all the Monk AOE spells, taken from http://us.battle.net/d3/en/class/monk/active/.
Note that I'm only listing AOE skills that hit a bunch of enemies at once (not Seven-Sided Strike or Tempest Rush), only runes that increase damage or lower spirit cost, and not listing spirit generators.
Skills, listed in decreasing dps:
Of course, this is very situational. Some of these spells affect a large area and will make it easy to hit a dozen monsters. Some affect a cone or path and will make it hard to hit more than 2 or 3.