... would be having the guy stand at the center of an explosion that would be big enough and quick enough to vaporize their brain or at least their brain-stem.
Short of that, a carefully-aimed sufficiently-large-caliber bullet is probably the quickest most humane death.
Unfortunately, the "whatcouldpossiblygowrong" quotient of using explosives is way too high and the margin for error in aiming the gun for the "perfectly humane" shot is also much greater than zero.
In both cases, there is also the violation of the moral rights of the condemned person's family to give the person a burial looking as close to life-like as any other corpse. In other words, the state shouldn't unnecessarily disfigure the person's body.
By the way, for execution purposes I would consider "instant death" to be "perfectly humane" when it comes to executions. Yes, I know that some other forms of death last long enough for brain endorphins to be released, giving a supposedly-more-pleasant death. And yes, when I die I do hope it takes long enough to get that endorphin rush. But if we are to have executions, those being executed are entitled to a humane, as-painless-as-possible death. They are not necessarily entitled to go out on an endorphin high.
For the sake of argument let's assume that the person is guilty of a capital offense and according to applicable laws qualifies for the death penalty. I'm not going to get into the obvious inhumanity of executing someone who doesn't deserve to die nor am I going to get into the argument about whether capital punishment is inherently inhumane or not.