if you're playing against good players who know how to check for spy (iirc you can go spy, disguise as enemy spy and check the 3d player model thing) i think the stock is best because they can't tell whether you're on spy or not
with that said though, the only time the enemy team will be able to do that is if they're holding their own last (or for some reason somebody walks back to spawn) in which case they can already change class or hit their resup bind and then are able to see you even when invisible because of the team outlines you get ~10 seconds after you spawn (which im pretty sure can be refreshed with the resup binds/swapping classes)
the big earner is pretty dumb because after you get the first kill you can suddenly run faster than a medic which makes it a lot easier to get more than one kill and odds are if you're taking any damage on spy you're kinda fucked regardless so having 100 health vs 125 doesn't really much matter
l'etranger and c&d are good if you're pushing last or in a stalemate and have time to wait
amby if you're looking to get that fresh killing double deagle med pick