That depends on what you are shooting. For any bullet-based gun, the answer is obvious. That will get through a skull with no problem. That said, real-world snipers never aim or the Hollywood-style head shots. Real-world snipers aim for....
1: Centre mass, the chest. Even at a mile away, a .50 calibre round impacts with the same kinetic force as a 9mm at point blank. Anther factor is the temporary wound cavity, which stretches the impact area. That is why the reverse side will always be larger than the impact side. A 7.62x39mm round is capable of inflicting a temporary wound cavity larger than the body itself. So, no matter how much everyone in Team Fortress 2 WILL complain about your body shots, that is where a sniper is supposed to aim.
2: Or a sniper will aim for the "fatal T," which is where the spinal cord meets the skull.
3: There is a factor called "cranial evacuation." This is an extension of the factor that causes temporary wound cavities. Basically, the skull acts much like a watermelon. If you shoot it with even a .45, the kinetic force will cause it to explode. But, instead of being some fleshy mass, the head is encased by a hard skull that cannot stretch or flex. So, that means the skull simply remains in that fractured state.
Needless to say, any of these will result in instant death.
Now for much less-powerful ammunition.
An arrowhead works best when it is wider than the shaft of the arrow. That will not only cause a temporary wound cavity, but it will also allow open air into the wound once the cavity collapses. For maximum effect, the best place to aim is the heart, but the lungs will also work if you don't mind waiting for your victim to suffocate from a collapsed lung, or drowning from blood filling into the lung. Aiming for the head is not a realistic option at all, but it could work, if you have a bow powerful enough to get through a skull.
Metal Gear 2 had that tranquillizer gun, which worked faster if you scored a head shot. That never added up for me. If you are shooting some sort of toxin, then shooting that at a solid bone would never do the trick. For a toxin to work correctly, it needs to be in the blood stream. But, you obviously do not want to hit an artery if you are simply looking to poison.
So, where you need to aim all depends on the ammunition, and what you plan to do.