Well, "fastest" along with painless was specified. The prompt gamma rays from the nuclear blast would kill all tissue within a very small fraction of a second -

Well, "fastest" along with painless was specified. The prompt gamma rays from the nuclear blast would kill all tissue within a very small fraction of a second - much faster than the comparatively slow C4 shock wave. :-) 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.