From the brief research that I have done, it appears that the executioner wasp sting is more painful, though the bullet ant's sting last longer.
The wasp sting appears to do notable physical damage to the tissue itself.
Posted February 3rd
I have never been stung by either a bullet ant nor an executioner wasp.
From several lectures I’ve heard the most painful sting comes from the Mexican asp.
At least two lecturers in different states said “At first I was afraid I was gonna die; and then I was afraid I wasn’t”.
Seems pain isn’t the only unpleasant effect.
In my personal experience the worst sting I’ve felt from an organism acting alone was from a scorpion.
I’ve been cluster-stung by yellow-jackets; maybe that was worse.
Bees, bumblebees, and ants, have also stung me; the bees in single combat, and the ants tried to Zerg me. But those weren’t worse; bad enough to avoid, but not bad enough to stay home from school or work.
I have to wonder whether a stonefish or a lionfish wouldn’t sting more painfully than the bullet ant or the executioner wasp?
I count myself fortunate that I will never know!
Edited February 5th