Medic (6v6)
The Medic is most commonly thought of as the most important class in TF2. He leads the pushes in order to cap more control points and to kill the enemy team. The medic's job is to provide heals to his teammates as well as provide them with overheals, also known as buffs.
Mediguns
In 6v6 the medic will only switch between 3 mediguns, the Medigun, Quick-Fix and the Kritzkrieg. The relation between the effects of each's ubercharge and building rates creates a rock-paper-scissors effect that keeps each balanced. Kritzkrieg beats the Quick-Fix because it puts out burst damage. The Quick-Fix, arguably, beats the Medigun because of the speed it can build at and because it helps a team stay alive for the post uber exchange fight. Medigun meanwhile beats Kritzkrieg because you take no damage while ubered but does build the slowest.
Medigun
The medigun has the ability to overheal the healing target to 150% of their normal health. The charge builds at 2.5% uber per second on a healing target without a full overheal, once the healing target's health is at 142.5% of their base health the building speed is reduced to only half. The minimum time it takes to get the medigun charge is 40 seconds. Once the medigun charge has been activated the medic and the class(es) on the receiving end of the healbeam will flash in their respective team color and will be invincible for up to 8 seconds. The medic can switch his ubercharge to as many targets as he or she would see fit to make them invincible. However that will reduce the total time of the ubercharge as the original target will retain the uber in addition to the new target for a short period.
Benefits: The medic and their healing target are safe from any kind of damage.
Weaknesses: Once the uber charge has been activated classes that are being healed can not capture control points, including the Medic. Even though the ubered classes might not take damage, knock back will still apply and explosives, airblasts and sentry guns can prevent the ubered players from entering an area.
Kritzkrieg
The Kritzkrieg has the ability to overheal the healing target to 150% of their normal health just as the medigun. The charge builds at 3.1% per second on a healing target without a full overheal, once the healing target is above 142.5% of their base health the building speed is reduced by half. The minimum time it takes to get a kritzkrieg charge is 32.25 seconds. The Kritzkrieg charge grants critical hits to all attacks done by the player who is being healed. The medic's preferred Kritzkrieg target is the Demoman because of his ability to cover large distances by charging his sticky launcher, big splash damage radius, big clip size and his high damage potential. During the time that the Demoman has the Kritzkrieg charge, his or her most important target is to kill the enemy teams medic and after that to kill as many of the enemy team as possible. If needed then Soldiers can also be kritzed. Scouts are generally considered bad to kritz as they cannot put out the same high amounts of damage with it.
Benefits: The Kritzkrieg builds faster allowing the team with it to increase a potential advantage or even create one. Kritz allows for killing the enemy team quickly.
Weaknesses: Activating the charge will not make the medic or their target invincible thus they will still need protection against incoming damage and planned attacks from the enemy team. Sentry guns will not take critical hits which makes Kritz hard to push into lasts with as the enemy team will likely have enough time to build one. The inability to become invincible means that the enemy players can easily ambush and kill the medic at any point.
Quick-Fix
The Quick-Fix has the ability to overheal the healing target to 125% of their normal health. The charge builds at 3.1% per second even if the healing target is fully overhealed. The time it takes to get a Quick-Fix charge is 32.25 seconds, the same as the Kritzkrieg when building at the maximum rate. The Quick-Fix heals 40% faster then the medigun and the Kritzkrieg which makes the Quick-Fix very useful after the initial uber fight has ended. Once the Quick-Fix charge is activated the medic and the healing target heal 3x as fast as normal. The medic and healing target will also be invulnerable to knock back, stun, slow down and air blasts.
Benefits: The medic will take the movement done by explosions to their heal target. This allows them to stickyjump and rocketjump with their team as well. The medic will also move at the same movement speed as his healing target. The Medic can heal players at roughly 100 hp per second or up to 300 with critheals when ubered. The medic is also buffed during the uber to 188 and receives the same heal rate as his target.
Weaknesses: Activating the charge will not make you invincible, damage that is dealt at the same time or within a short period of time will kill the Medic quickly!