Twilight Vortex would be a nice place
The Whole of Sholizar Basin.
When you are up against a Facey situation, don't forget to use your Face.
I would take the tree's out of the Temple of the Moon in Darnassus, put a circular bar on the place of the moonwell/statue and there you have an awesome party room.
Picking a room without any modifications made to it then it probably would be the Grim guzzler. That's just an awesome bar.
My guild turned the Stormwind Station of the Gnomish Underground into a Rave.
see the video http://www.veoh.com/browse/videos/ca...203013W9j686qg
The last room in the Throne of the Tides IMO....just without Neptulon and the giant squid that wants to eat you.
The Den of Mortal Delights.
Kel'Thuzad's or Sapphiron's room.