For highest damage in a single shot? I'm thinking a headshot from a bolt action sniper rifle. Great for picking off humanoid snipers and rocket launchers from far away since you can one or two shot them.
For hellbug arkfalls sniper rifles suffer because a crystal or boss can easily survive a single shot, things are moving all over and you can close the distance with targets to use the pistol.
Sniper rifles may be worth it on scapper arkfalls to save time running from one arm to the next. Shoot one down,
aim across the map at the next. That's a possibility. (haven't tested that yet 
)