Originally, my favorite was inert gas asphyxiation. Based on previously found info, it seemed like the best option, just falling asleep and dying. Of course I was aware of the main risk: running out of the inert gas and again getting oxygen. That could result in permanent severe brain damage.
However I later found even worse stuff. Veterinarians apparently stopped using this method as the animals were showing severe signs of distress, meaning that it perhaps wouldn't be as painless.
Finally, there was a nitrogen hypoxia execution in Alabama recently:
Smith appeared to remain conscious for several minutes. For at least two minutes, he appeared to shake and writhe on the gurney, sometimes pulling against the restraints. That was followed by several minutes of heavy breathing, until breathing was no longer perceptible.
They fucked it up, he held his breath and caused himself pain because of course he didn't want to die and it's barbaric to kill people. But also, those spams that happened are after you have already lost consciousness
There's no such thing as a nice peaceful death. I've witnessed what people medically consider the "die peacefully in your sleep". People still spasms and throw fists and kicks after losing consciousness. It usually just happens when no one is watching and lasts only for a minute or so while the brain stops working completely.
It is, but it doesn't look like it from the outside. The body still moves and trashes about for a while after consciousness has presumably faded. Hell, bodies sometimes sit up on gurneys before embalming.