You want technicality or actual usability?
Technically, it's No Land Beyond. Guaranteed one shot kill if you land a headshot.
In actuality though, The Last Word is the fastest killing primary in the game. In fact it's so fast that it's almost impossible to react to.
Actually that goes to universal remote and no land beyond since they can technically one hit kill. Otherwise it's last word since it has issues at the moment.
Definitely thorn for guaranteed damage. Everything else relies on perks that may or may not activate. Thorn has WOC speed with fatebringer impact, AND has damage over time. Moral of the story, nerf thorn.
The Last Word. It's been proven. But it takes some serious skill to get the head shots while firing from the hip. It still beats Thorn even when aiming down the sights.