I get that, but I'd like to know who is more important in a teamfight, their carry or their aoe stunners and such. Should I focus FA or Witch Slayer if they're both the same distance away. I tend to focus their supports first and let my teammates/str hero tank the carry.
Focus ws, but make sure someone interrupts fas ult when she uses it.
There are many different variances to take into account when approaching a team fight. To generalize, the supports should be taken out first if the opportunity arises with disables being played to counter various ultimates and abilities. In a perfect situation, the support should fall first, the carry second and the tank last. It's incredibly easy to say this, but because so many factors come into play that determine a team fight's outcome, focusing on line-up should not be the priority.
Many 5v5s outcomes are entirely dependent on positioning and reaction timing. If you are a support, do not over-extend to a position where you can be reached easily, if you are an initiator know the time to initiate and if you are a carry you'll need to be able to put pressure on targets that threaten certain team-mates.
I think what Rhayne has been trying to say, is that you'll need to start weighing up in your head what poses the greatest threat to your team while keeping in mind the above information I've laid out.
All in all, it comes down to experience with heroes, items and positioning. Practice makes perfect, as they say.