In terms of cost effectiveness? Anti-Material Rifle. It's only good for Deathclaw hunting. :shrug:
Actually, per base cap shot, the Gauss Rifle eats up more. 3 caps per MFC * 5 MFCs per shot = 15 caps to fire that thing.
One .50 MG round costs 6 caps.
Now, if you get into special ammo, then the price per round goes up. But then again, Over/Maxcharge goes up too once you factor in how many regular MFCs you have to expend to make them.