tbh, I'm gonna go outside the box, and say that the fastest 1 shot tech is the human path. before you all jump down my throat, KA is a genjutsu that, while powerful has too great of a charge time to really be considered a 1 shot tech, and we've no idea if you can make someone kill themselves with it, seeing as shisui was overpowered by danzo. Kamui while also good isn't as instant as ppl think, kakashi has to activate his MS and gather the required amount of chakra for the tech, once the chakra has been gathered however, he is fairly instantaneous with it. if a fast opponent is able to put pressure on him before he uses it however, he can't activate his MS and gather enough chakra and keep avoiding his opponent.
Human path however is the touch of death. I know due to plot many ppl think it weak, but barring plot, this tech is arguably one of the most dangerous. Using it in conjunction with the divine pull is enough to 1 shot even the likes Naruto. not even an edo is safe from it, as one can simply pluck a soul from its shell. IMHO, no other tech is as deadly seeing as unless you seal edo's they'll come back at ya. Human path makes them childs play.
