I would say the Cemetery of the Forsaken.
With a chance for a unique to spawn, plus three very small dungeons, two of which are one level only with exit teleporters, you should be able to find 5 stacks in a very short amount of time.
Another very good area would be Festering woods. The area is quite small and usualy has 3 elite groups by itself. There also are 2 very short dungeons with 1 elite group in each.
A lot faster than cemetery of the forsaken since the area is much, much smaller. However, the trash deals more dmg, so you have to have a bit better gear.