35k+ healing is what you should try to hit every game. (However this is mainly building so dont read too much into it)
As few deaths as possible obviously.
Deaths after charge is a underlooked one it shows your post uber positioning which is very important.
Avg time before building is probably the most important. You want this to be as small as possible. Under 60 seconds is what the top medics hit in prem hit consistently. If you see a medic hit like 45-50 thats really good. Think June hit nearly 52/53 seconds in polan GF.
Deaths near uber is important as well it shows the uber tracking is on point. If you see a medic die a lot near uber they messed up on the tracking.
Advantages lost too
HubertTheFishadvantages lost has no meaning whatsoever. if you use uber as soon as you get it you will have 0 advantages lost. if you take ground off of ad it will count as an advantage lost when the other team gets uber. do not look at this stat.
And obviously kills.
Don't fall into the trap of damage taken being the most important.
So to answer your question:
A good medic performance will have not many deaths, few drops, few deaths after charge, 30k+ healing, 0 advantages lost and a low time to build.