When I’m the team’s Medic, I have a golden rule for choosing which teammates to heal:
“All heal targets are equal, but some are more equal than others.”
Unlike other classes who can remain effective even without a clue as to how capable their teammates are, the Medic is wholly dependent on his teammates being able to intelligently use his heals to accomplish objectives, kill teammates and keep the medic himself alive. And that’s going to require the Medic to serve as something of an arbiter, judging his teammates and deciding who gets special treatment. It is impossible to play Medic and heal everyone equally, as much as our altruistic natures impel us to.