As a mvm spy you can choose 2 playstyles:

The Money Spy( Replaces a Scout )

The Crow Control Spy aka medic slayer ( replaces demo and sniper )

Depending on your playstyle you should choose a correct set of upgrades.
If your playing as a money spy you should look forward into getting mobility and resistence upgrades.

If your playing as a crow control spy you should look forward into upgrading your knife and sapper.
Also Never NEVER upgrade your revolver, its not your primary weapon in a mvm match.

Remember: your best friends are the dead ringer and the knife and the sapper.

Well since your best friends are the knife and the sapper if you look forward into having a trusty revolver we have some choices.

The Ambassador: Its good if your also trying to kill support classes like snipers and razorback snipers but shouldnt be a main choice

The Enforcer: Good for tank waves but not very effective at killing robots because of the slower fire speed and that the bonus of 20% damage is only applied when you are undisguised which you should never be.

The DiamondBack: Good for tanks waves in general but not very effective and not much used.

The L'Etranger: best all around the revolvers mainly for the crow control Spy since you need survivability and damage overall you can use the L'Etranger to hit small targets like scouts and get more cloak if you dont have any med/ammo pack around.

The crow control second best friend.

The knives are the main damage dealer of the spy, so lets start with the ones you should throw in the trash and forget they were added to the game:

Your Eternal Reward(YRE) and Wanga Prick: Dont even mind using that in MvM. the lack of a disguise kit will make you a instant target for the hordes of scouts with their

The Kunai: It's decent but not the best but the hp penality ( -65 health) makes you a useless kid coming out of spawn every minutes.

The Spy-cicle: DONT EVER use this. If you ever get hit by a single flame this will put you out of combat for 15 seconds until you regenerate your knife. this means 15 seconds that you cant backstab medics that will activate uber and break your defense and probably your sentries.

Now that we covered the useless knives lets start with the amazing pieces of gold:

The Default Knife or Black Rose: primarily your best friend good all around no down sides, nothing more to say.

The Big Earner: kinda like the kunai but a little more useful since you get more cloak, but the health penalty is a serious thing.

As tips i want to start with what most of the spies forget:

Don't ever go near pyroes, if you ever get hit by a flame every robot in the entire map will know that your a spy.

Dont mind choosing a custom sapper in mvm it doesnt make a difference. ( but the AP-SAP talks a ♥♥♥♥ lot in MvM tho)

Always when you have a sapper and there is no giant or medic treats in the map always try to sap the sentry buster. The sentry buster and the Uber are the only things that can break your defenses.
( i recommend using a combo of fan'o'war + sapper in the sentry buster for better results )

Never waste sappers at hordes if there are sentry busters , giants or medics in the wave