The Demon Hunter class achievement Right Where I Want You requires you to "Keep a boss snared using Caltrops for 40 consecutive seconds."
Since The Butcher, Belial, Azmodan, and Diablo are all act bosses, they would obviously count, but what about the sub-bosses The Skeleton King, Maghda, Zoltun Kulle, Ghom, Siegebreaker Assault Beast, Cydaea, and/or Rakanoth?
Which enemies count as a boss for this achievement?
Obviously once one person gets the achievement, they can't get it again except on another account with a different copy of the game. This problem is more an exercise of information gathering using outside sources and friends or at the very least one good community wiki answer to which multiple users contribute.
As a subsection of this question, which of the enemies that count towards this achievement would be the easiest to get this achievement against?