The next part of the medic guide was supposed to be medic matchups, but I figured it's more important to talk about the all-too-controversal healing order. So many medics deny that there is such a thing, but there is a benefit to understanding how each class benefits from overhealing and healing. There is no set heal order in 24-32 man games, and you should overheal everyone always, but sometimes before battle and in the thick of it, choices have to be made. Neglect NO ONE. Heal as many people as you can, even if you can't get them all to full health, spread the love. Remember pocketing does not mean ignoring 99% of your team.
I start with sniper because most medics never think to overheal snipers, but sniper is actually one of the most important classes to overheal just because of how sniper vs sniper plays out. A good sniper moves around and quick scopes and gets quick scoped. A regular headshot quick scope does 150 dmg, more than the regular life of a sniper. With full overheal, a sniper has 185 healh, more than enough for your ally sniper to get 2 shots off to the un-overhealed enemy sniper's 1 shot. Overhealing and full healing will also give your sniper more confidence to keep poking out and taking shots and moving up to get into the thick of things... if they're smart at all.
For the love of the holy banana, heal other medics first. Always pay attention to the health of other medics. As a medic, you should hover over your teammates' health status, but ALWAYS heal another medic over any other class, because hey, they can heal the offensive class while you heal them. And if your offensive pocket is being dumb, retreat while healing the other medic and live to give your uber another day. Don't hesitate to yell at the other medic, “HEAL ME YOU PRICK!” or “MEDICS HEAL OTHER MEDICS FOR LEBRON'S SAKE!” Remember that medics work well in pairs in the same vicinity. Medics working together spells certain doom for the enemy.
Heal engineers. Heal suicidal combat engies. Heal engies building dispensers. Heal engies repairing their sentry under attack, most of all. Medics and engineers (their buildings) are prime targets for the enemy, and with good reason, they present choke points. Keep the engie alive so he can keep banging on his sentry. This works especially well in uber heavy/soldier/pyro situations where the enemy may or may not be able to destroy the sentry easily. And by heal I mean keep at full health or uber if you need to.
Rocket jumping takes a lot out of a man, so at the end of the day, I like to relax with a nice can of Soldier overheal. It really takes the edge off. Overhealed soldiers can make 2 or 3 extra jumps before getting worried about health, not to mention the obvious damage mitigation.
Demos do jump, but more importantly smart demos know that pushing with stickies is risky but effective. Demos also need to be overhealed and healed to keep pumping out spam. Remember kids, spam wins games.
Overhealed heavies survive much longer in large firefights, but more pertinently, the extra health might allow the heavy to run forward through the hail of fire and get close enough to minigun at point blank. Smart heavies get close. 'Nuff said.
Sure, bonk makes em invincible, but scouts need love too. They're super quick to heal, so why wouldn't you? Plus, the more health they have, the better flanking position they can reach.
Overheal pyros going into the battle so they can do their reckless job. Keep pyros airblasting spam alive, because they're keeping you alive by airblasting. Pyros enjoy the overheal, and can save your ass from spies or anyone trying to get up in your grill. Rock 'em.
The only reason they're around you and not behind enemy lines is they're probably looking for health. They heal quick, so get them back out there backstabbing.
Bottom line: There is no set heal order. Overheal everyone constantly and remember to keep other medics alive. If you can constantly switch and heal and overheal while being supremely mindful of your pocket's health and keep fellow medics alive, you sir, will be a good medic.