I'm gonna cheat and pick two.
Knock Knock the named fire axe is so strong that you can steamroll pretty much anything by the time you get it, not to mention that the vats special attack knocks people over and launches them several meters.
The Abilene Kid LE BB Gun was also godly. It didn't do much damage, but it shoots fast, the ammo weighs/costs nothing, stored 100 shots a 'clip', and all critical damage took at least 50% health from anything in the game making for easy crippled limbs. Pair that with a sneak attack to always get that critical or cripple and you're set. Also pretty easy to get as long as you are nearby Nellis.