The most obvious reason to favor certain players is depending on their chosen class. “Pocket” classes are obviously better at defending Medics and several, such as Demoman and Pyro, have tendencies to lose their health quickly alone. A continuous healbeam can allow them to take their offensive capabilities to the next level. Be wary of their loadouts, though, because many unlocks turn a good pocket into a bad one, like Demoknight weapons and Soldier unlocks that have to do with rocket jumping. There aren’t many unlocks that turn a bad pocket into a good one, though.
You should also factor in the cosmetic getups of your teammates. I know, it’s generally frowned upon to assume the guy with the gold-painted unusual hat is more skilled than the naked guy, but as a Medic, you are putting your life in the hands of the people you heal. When you get ambushed and have to decide which player is more likely to respond to your cries for help, you’re going to have to play stereotypes and make some possibly rude decisions about your teammates. Awkward Zombie says it best in the description for this comic.
Finally, and most importantly, a Medic must factor in observed player skill. It doesn’t matter if a Heavy has a Sandvich, an unusual, and every possible opportunity to be the perfect pocket; if the player at the controls doesn’t have the skills to pay attention to your needs, or if he honestly isn’t interested in protecting a Medic, you have to ignore him. I’d say this point is almost more important than the last one; if given the choice between sticking with a low-skill oblivious Demoman or a Scout who’s proven repeatedly willing and able to protect me, I’d take the Scout any day. A Medic can help any class excel, and while obviously we’d prefer that our heal targets be the appropriate heavy-hitters to get the job done, the battlefield is rarely so accommodating.
But at the rule states, all heal targets are equal. You really should keep everyone topped up as often as possible. For one, spreading the love around builds your Uber faster and keeps more bodies between you and the enemy. For another, even players who aren’t ideal pockets will usually want to keep you alive just because it ensures they keep getting occasional heals. And of course, a team of fully-overhealed mercenaries is going to kick a lot more ass than a team of battered mercenaries with a Medic glued to a single player.
Most of these rules go double for deciding Uber targets. When invincible, skill becomes just as important as class decision because your target is only going to get 8 seconds to deal as much damage as he can. Given the decision above between the Demoman and the scout, I’d have to look at the situation to determine my choice. Am I hoping the Uber will destroy a level 3 Sentry Nest? I might end up Ubering the Demoman, begging him over the mic to please focus-fire the sentry first. If I’m merely hoping to turn the tide during a control point fight I might Uber the scout instead. In Capture the Flag, remember that a player holding the flag will not turn invincible when Ubercharged with the stock Medigun, so it might be better to Uber a nearby teammate to defend the flag carrier.
Also, sometimes you might have to make the decision not to pick a heal target and run for your life instead. As the team healer, keeping yourself alive should always take precedence over sticking around to try and save your teammates. A dead Medic isn’t building Uber and he isn’t healing anyone. The act of leaving your heal target for dead might go against your altruistic nature but if he was a good pocket he would be willing to sacrifice himself to keep you alive.
For a class that defines teamwork like no other, the Medic can actually be a pretty selfish team role. Just remember that you control who gets healed and reward those players who show they care about keeping you alive. If you’re interested in refining your medic skills further and perhaps exploring other aspects of playing Medic, like self-defense and mastering his many primary weapons, this thread on the Steam Powered User Forums contains links to a number of very interesting and useful guides. And always remember the other golden rule of playing Medic: “Pop it, don’t Drop it!”