Now, I just want to clear this up. Some people claim this or that is the most powerful in the game, however I know the weapons that completely outclass all other weapons. No matter what the level the enemy is, these weapons can instakill that enemy.
It is simple, the most powerful weapons in the game are...
Sheev Karyst Heat Dagger Ceramic Dagger Dark Dagger
Assuming the enemies scale endlessly, all weapons EXCEPT daggers will eventually fall off in damage.
There is one reason for this...
Ok I'm done. Go get yourself one of these weapons and the Covert Lethality mod and win the game.
In response to some comments: Go into the simulacrum and try to 1 hit kill a level 100 Drekar Manic Bombard. You won't. I have tried the Tonkor, the Synoid Simulor, the Lanka (red crit on a Sonar spot), and more. The scaling on some enemies is just ridiculous. In the end, the only way to consistently 1 hit kill an enemy is using a dagger with Covert Lethality. Also, by stunning an aware enemy with Paralysis or a similar ability you can still get the instakill bonus.