Even though I have around 600 hours on sniper (and I REALLY love stock sniper rifle and kukri), I would have to go with all spy knives, except your eternal reward and the spycicle, I dont know why, they're good but I just don't think disguise is viable and as a knife spy being punished for getting too close to a pretty common class (as a knife spy), losing my knife is not ideal.
If I had to pick one weapon, Conniver's Kunai, handsdown, risk/reward, really helped me learn how to play knife spy (granted I'm bad at it, but I'm better than I am as gun spy). As possibly the weakest class, being able to sustain my health is so good, absolutely love pairing it with stock cloak watch and L'etranger.