I probably shouldn't be giving advice on medic since I think battle medic is the most fun class in TF2, but nevermind, I play a lot of medic.
Primary - Crusader's Crossbow
Yes, really, I'm serious. The crossbow. Most people think the crossbow is useless. They're absolutely right it's underpowered and difficult to use, but it's not useless, and certainly not compared to the alternatives, which are underwhelming at best. The crossbow has a very definite feeling to it at long range, whereas the alternatives feel clumsy in any situation.
Note that it's unusual in that damage is inversely proportional to range (most weapons are either constant or suffer drop-off, the crossbow is the opposite). It does a maximum of 75 damage, which isn't much, but it can take down a sniper in two shots. Snipers make fairly easy targets because they'll scope in and stand still while you can jump around like a rabbit with ADD, *and*, because of the very very precise X-axis accuracy, still shoot straight.
The bolt follows an arc so you have to aim above your target, but you get the hang of it easily. The bolt itself is relatively slow moving, so you need to lead your target at range, if they are moving, but again, this isn't too hard.
It actually has extraordinary range. E.g. In Thunder Mountain stage 1, you can easily snipe BLU's snipers at their base exit from the balconies overlooking the ramp. It's also good for finishing off people running away. And for sniping engineers building sentries.
The other factor you need to consider is the huge number of crits a medic gets. Crit chance in TF2 is proportional to how much damage you've recently inflicted, and if you're a medic, you inherit the damage inflicted by your healing target! And a critical bolt is a serious threat to anyone.
And of course, finally, the crossbow heals teammates. Fast, from any range. But unfortunately it doesn't charge your uber.
I don't think there's too much to discuss about the mediguns. You should default to the stock medigun. You might prefer to take the Kritz if there's another medic on your team (wielding the stock), and if you have complimentary classes to use it on. Whilst heavies make great ubercharge targets with the stock medigun, Kritz charges are generally effective only on demomen with sticky launchers (these are an absolute beast when kritzed!) and soldiers with a 4-shot rocket launcher (a heavy is still _okay_, but not great).
The quick-fix is rarely helpful, in my opinion.
The melee choices are more open to discussion.
Stock bonesaw (and equivalents)
There's no reason to use this over the amputator. The amputator has identical stats, and does more additionally.
The ubersaw gives you 25% ubercharge on hit, which is great. If you can pair up with another medic, you can literally keep cycling between ubering your friend while he attacks with his ubersaw, and then when yours runs out, he should be able to uber you. Repeat ad infinitum. It's a very effective tactic, until a pyro comes along and airblasts you. But for normal play, I prefer the amputator.
The VS retains up to 20% of your ubercharge when you die, but I don't really find it that useful. 20%'s not a huge amount. If you're dying that quickly then that's bad news anyway, maybe the 20% will help a bit, maybe switching to a faster charging medigun will help (although obviously the ubercharge will have a different effect), or maybe you just need to be more careful. It's situational, although I rarely use the VS.
The SV lets you see enemy's health. This makes a lot of sense if you're using the crossbow, as you'll know who you have a chance of killing.
The amputator is my personal favourite. Coupled with the crossbow, you get a health recharge bonus which gives you great survivability (particularly against pyros, and similar after-effect damage).
It also has a useful taunt (press G), which has an area-heal effect. You can often stand behind a corner and group heal most of your team at once. Sadly this doesn't build your ubercharge, but it's helpful to your team.
Take the stock medigun, crossbow, amputator, and take any good crossbow shots you are presented with.