My friend suggested a quite humane (in the sense of no pain and little discomfort) execution method, which is unfortunately also extremely horrific: strap the condemned person flat on a platform in a small room and then drop a large 1000-ton block onto them from a great height. It's basically guaranteed to completely annihilate them in a matter of microseconds, which is faster than the pain can register. Apart from a small nuclear or fuel-air incendiary bomb, there's not much guaranteed to be that fast.
But of course, it would be too horrible to implement. Personally I don't see why we don't use an inert gas, optionally combined with a mild sedative or euphoric, to displace the oxygen in the room. It would be more pleasant than poison, which we seem obsessed with. I think we just have a psychological need to administer a killing blow; painless asphyxiation is too close to mere deprivation.