Stock+reskins: The most reliable, pretty versatile. It gets the job done well, but I don't think it can take you far like the kunai can.
Conniver's Kunai: My personal favorite that I have an extreme love-hate relationship with. While it requires good sneaking/flanking and some luck from stray spam, it can be very rewarding for spies that can trick stab. Imagine that pyro charging at you only to be matador stabbed, feeding you up to 180 health allowing you to kill that scout who thinks he can mess with you. One of the greatest feelings I have had as a spy.
Big Earner: The pro of this really doesn't outweigh the con, as it's bad for the IW and DR if you know your ammo boxes(plus the weapon you pick up from the person you backstab) It compliments the CnD very well, but I don't even find the CnD useful.
YER/Wanga Prick: I really think its only worth it with the Saharan Spy set, which is just easy mode. Since the Wanga Prick isn't compatible with that set, I totally disregard it.
Spycicle: I like to use this for giggles on my combat spy loadout- Ambassador, spycicle, DR. Other than that, I see it as a kunai with a passive effect. Also takes your strongest weapon away for 15 seconds.