There's lots of Warlock jokes (my favorite is the thought that the first Warlock with a 41-point Demonology talent will be the most powerful in the known Azeroth universe), "any pally for 8 seconds," the expected suggestion of Arthas the Lich King, and even a mention of High Overlord Saurfang (who, with that link, might replace Captain Placeholder as my new favorite NPC). In the end, the debate basically comes down between Sargeras on the bad side (he's a fallen Titan who's become the main villain in Warcraft lore) and Elune on the good side-- even though her identity in Azeroth as the "God" of the Night Elves isn't quite clear even inside the Blizzard offices. Sargeras, however, is currently trapped in an "unbeing" state, so the title probably goes to the Titans, a group of beings who oversee the Universe (and eat old gods like C'thun for lunch-- the Master's Glaive in Darkshore is a remnant of the Titans' power).
Of course, as Drysc points out later in the thread, Chris Metzen is really the most powerful being in the Warcraft universe. Way to ruin the fun, Drysc.