Honnestly, as a main MCH : The target closest to me or the one I can uses AOE on it. And I'll swap my main focus/damage to match the party's current target. Yes I'd like, in a perfect world, to have a list that I can stand to at any time, but the reality is that you need to adjust to your team and the situation. I try to put marker too, it helps on focus.

The important thing I learned in CC : Focus a target with your team. And if it runs away to get an elixir or heals on the maps, it's pointless to chase and leave your teammates behind, it's better to switch target, because you just turned the fight at your advantage being a 5v4 for 5-10 seconds. If it's a kill it's even better.