I don't think that guillotine is the fastest and most painless...
Why? The body's nerve receptors all go through the brain, and the body and head are seperated.. Feeling no pain and instant death, that's humane right? The actual method is ratherly barbaric, and messy, I suppose the overall method is not to humane, but it's 0-death in less than a second.
it is not clear whether the guillotine was as painless as it was intended to be. i would much rather go for some delusional method where you can't even tell you are dying.
if none really exists, then just to be laid down on some giant metal plate and be squished by another giant plate in a split second--the human pancake maker.
How about natural causes From old age in prison?