The Fastest and Most Painless Way to Die
What is the fastest and most painless way to die ?
Nothing faster, period - and no time for pain impulses to travel along nerves either.
In practical terms though, not much difference. If practicality was the major concern I'd say a 12-gauge discharge using #2 shot or so through the forehead angled down towards the back of the head would work well. Messy cleanup, but quick and painless for the deceased.
But the best way to knock the brain out is other.