Lead poisoning is cheap and effective every time. And when applied twice daily to the head, painless as can be.
While I agree with the sentiments expressed, I fear that exhibitions such as this will lead the courts to ban death penalty. My solution is nitrogen. It is hardly toxic or difficult to come by, 80% of what you are breathing now is nitrogen.
BUT place the soon-to-be-deceased in a sealed chamber and flood it with pure nitrogen, no oxygen, they won’t notice. No flailing about or collapsed veins, they just go to sleep and never wake up.