With the repeater regions, you'll have something like Grid or Healbot or Vuhdo.

I know with Grid, it colors the box more as a toon is taking damage.
You could set your default heal to a left click.
And another heal to a right click.

I personally like to use modifier clicks, so I can select or context menu the toon, when I'm on the healer.
But Grid needs Clique, which modifies all of your clicks.
Healbot and Vuhdo interpret clicks on their frames, still letting you click the party frames.
So it depends on the addon you are using.

It won't automatically target the worst character.
But it is very easy to see which box is more colored then the others.
And you just click that box with your mouse.