I'm going to say Kritzkrieg, and it is pretty logical.
Although I do not have it equipped, it is the best Medic weapon you can have. In my opinion, Medics are to help their team. Like they are listed, they are support classes. Like the Sniper and Spy, you can do without them, but why?
Blutsauger and Ubersaw only give reasons for Medics to neglect their team. Some "medics" are more interested in going to an enemy, killing them, and post "B-B-B-B-BONESAW!!11!1" in chat, while the one killed, and their team, is wondering why they walked off like that. The Ubersaw was only created and best used to either try to spike your ubercharge early by hitting an almost dead enemy or late when you almost have uber in heavy traffic in my opinion. Only if it's really necessary should you really whip out your bonesaw. Taking it out for no reason to chase some guy to type all that in chat is just a way to make everyone think you're bad arse, and no one truly cares but you.
The Blutsauger is in my opinion, more useful than the ubersaw because if you get in danger, this is a more logical weapon to use to flee, especially against Pyros. You are only a tad slower running backwards than a Pyro running forwards. Although...
The Kritzkrieg is in fact, the ONLY team weapon the medic got. This can help in a pinch in plenty of maps, and we take it for granted because we are considering only ourselves when it is used. We say "Well, it can't be much helpful if we can be invulnerable, but it helps more than you think. The weapon is best to take out enemies than sentries, and the Soldier is probably the best class for the job.
If you are able to find three medics, with two with a medigun and one with a kritzkrieg. Grab a heavy or a soldier with you, have a medigun medic uber the kritz medic, who is ubering the soldier/heavy, but have the other medigun medic ubering the soldier/heavy as well. Sure this looks foolish, but get those three medics standing in front of the heavy, and you'll be surprised how much goes down.
This is a TEAM game, after all.
/rant