My attempt at categorizing painless deaths just through my thoughts
A bullet to the head: painless aside from the psychological aspect, the brain has no nerves to signal pain and the human will only be alive for at max a couple milliseconds, although this could change depending on where the bullet penetrates (lethality hardly changes, bullets are too erratic inside the body)
Choking: the human will feel fuzzy at first as oxygen stops going to the brain although unless there is quite a bit of fighting back or the strangulation is performed incorrectly (by pressing on the windpipe instead of the arteries next to it) I'd say it's relatively painless
Hanging: unlike popular belief you don't die from choking in this one, you die from your neck snapping, this is somewhat painful, especially due to the rope, but you would die pretty quickly after considering your neck snapped and blood is no longer pumping to your brain (barely qualifies)
This will be added onto when my head isn't splitting open, for now I will be working on concocting "pain"