In my opinion, it's stock. I feel like all the other ones have too much to lose, be it health, presence of a frozen corpse or the ability to disguise freely, stock really delivers the full package and let's you perform at peak. All other knives incorporate a different play style that isn't really transferable.
The Kunai requires a more passive play style and a majority of the time, the hp you earn from a backstab is going to be less than the Spy's base hp. Of course, the perks of consistently gaining hp is always a plus but, especially if you are just looking to start as spy, doesn't give you much opportunity to perform.
The YER sacrifices cloak for disguise which pairs well with the cloak and dagger but both of those also play an opposite play style and also doesn't allow for much room to practice trickstabs other than chains.
The spycicle allows for extra survivability in this Pyro dominated era but not for long as the frozen bodies give the enemy tank Intel that you're doing work leaving them much more paranoid. As time progresses and you become more confident in your execution, you can use their paranoia to your advantage and land some easy trickstabs. I advise using it later down the road.
The big earner gives you the extra cloak and speed with each backstab but speed component can be obtained with the dr and the cloak with the letranger. Of course, that consumes 2 of your weapon slots but those 2 weapons are very communally used as spy anyway because they provide whatthe big earner does and then some from a range.
It's really up to you which knife you use but be prepared for any consequences that come with them.