Anyone that thinks there is a set priority list is only gimping themselves and their team.
I don't think that's the spirit of this thread. The point is that there should be a heuristic hierarchy based on shutting down the other team's good players and disrupting their synergy. This is done by making their healer panic and neglect their team while also keeping him/her CC'ed. However, it's so difficult to get pugs to focus fire since they are like those dogs with the cones around their heads. And as someone else already mentioned, good luck trying to keep them from breaking your mezzes.