I've only recently started playing as Medic so I am still trying fine tuning the way I play.
I find this depends on the situation. If there is a big retreat from a pyro, I'll tend to put out anyone in flames first and then move on to look at who has been calling for medic.
When it come's to the situation where everyone is hanging back waiting to launch an offensive, I'll pick who I think is going to be the most effective with an uber. Generally you'll find most of the demo's, heavy's and soldiers will all be calling for medic in this situation. Obviously I can't run around after everyone. So I pick the player that has been most effective so far on offence.
If I am a few seconds away from the uber and we are about to launch an attack, I generally will ignore retreating players and concentrate on timing the attack. If the person I wanna uber is ready and I am off healing other people it could throw the timing off and make the attack a failure.
I think this very much depends on how you play as a Medic... Some stand back healing as people retreat, others like to head closer to the front line. Personally (even though it does me little good in the end) I like to follow closely behind the demo as he surges forward, dodging left and right. In that situation, its difficult to take on other healing targets as you are on a surging run forward. If you leave your guy who has been used to you behind him, its effectively a death sentence.
That said, if my guy is doing something stupid and I see someone else making a forward run, I will move to them.
It's important to have a little bit of selfishness as a medic. You're no good to anyone dead, so I will run away if I am about to die and I'm not close to an uber...
I'd actually like to say that on dustbowl I have had some great partners, seeing I am on my last legs they've made runs to a health pack for me to grab. I also really appreciate the passing medic healing me and I heal my guy.
As for the guys that constantly call medic over and over... Well they wont get any extra healing from me... If they are dying, sure you'll get a heal... but you will not get me partnering you in game. Especially if you yell over the mic "why isn't any ******* medics healing me"... In doing that you've already answered your own question.