Actually, instead of looking for specific enemies or enemy types, I suggest one special event:

The [Jar of Souls][1] will spawn a very large amount of skeletons, and you can also lure monsters from other areas of the crypts to the location of the event.

Other than that, the Carrion Nests come to mind, which can sometimes occur in groups of two or three and all spawn Carrion Bats, as seen here: 

![enter image description here][2]

They occur in Act 1 randomly in many of the random dungeons, as well as in the Fields of Misery and around the Highlands.


  [1]: http://www.youtube.com/watch?v=kdf711kPnWw
  [2]: https://i.sstatic.net/lWK0A.jpg