I think quick-fix might be the most underrated medigun
Demoman and soldiers that can access literally any part of the map from a long distance away is dangerous enough, but with the quick fix, they can take a medic with them and suddenly that soldier or demoman can hold their ground against a good half of the other team. The critical heal heals for 100hp/sec and ignore sentry knockback too, and a demoman can quite easily outlast a level 3 sentry in the time it takes to shoot a few sticky bombs while been critical healed by the quickfix charge.
I actually thought that I might see people using the quick fix as an attacking weapon in the way I've described, but so far everyone just plays it safe and boring. The medic could access the stairway leading down to reds resupply at the final point of bad water, the medic could follow a sticky jumping demoman all the way over and above the badwater tunnel leading out of blue spawn, and there are many other good applications besides these two examples. But, nope, all the medics just use it in the safest ways possible. How dull.