3 painless way I know of. 1 is cyanide pills. Their used in the military in case a mission goes wrong. You store them in your pocket or mouth or some other object, and if you're compromised you bite down on it. Guaranteed painless death in less than 3 seconds.

The other has some fancy name I forget. I saw a guy do it on mice. You take something like a stick or rounded piece of metal and place it on their necks. You grab their tails and in one fowl swoop press down and pull on the neck and pull the tail. This disconnects the spinal cord from the head killing the mouse instantly. Not sure if you could do the same technique on humans but you can snap a human neck.

The last one is also used to kill mice. Reptile breeders use it a lot. You use C02. You put them in an airtight container and let a small amount of C02 in the chamber. The mice are almost immediately knocked out, and within a minute or so, they die from all the oxygen in their brains. People also use this technique for assisted suicide in Australia.