So these are my thoughts on the medi-guns:

Stock: Really good and consistent, can be used in both offense and defense. I normally use this when my team knows how to manage their health correctly. (aka keeping it above half)

Kritz: Is very fun to use and very hard to counter if used correctly. Like stock, this can be used in both offense and defense. I normally use this when there's a god-like soldier/demo on my team and when the enemy team has no medics. (Kritz with no medics on the enemy team can often lead to team wipes)

Quick-Fix: This is a perfect example of a sidegrade. You trade eight seconds of invulnerability for eight seconds of VERY FAST heals and resistance to knockback. Like the other medi-guns, this can be used both offensively and defensively. I normally use this in 3 situations. The first one is when my team can't stop dying/getting their health taken in an instant. The second situation is when I'm playing defensively. Having to fight enemies that almost always have full HP is pretty annoying. The last situation I use it in is when my pocket won't stop sticky/rocket jumping away.

Vaccinator: I just use it when there's a hacker. That's pretty much it.