Disclaimer: Violence is stupid and dangerous. A blow to the skull is often fatal.
I wouldn't say it's easy, but there are several sweet spots on the skull that, when hit with sufficient force, is almost guaranteed to cause unconsciousness.
I would suggest the upper part of the jaw bone or the temple. Both places are connected to the brain stem through nerves and/or arteries. A punch here will cause the force to reach your brain and shut you down.
I also believe that it's an important factor whether you expect the punch or it lands like a lightning from a blue sky. The latter are by far the worst. You'll almost certainly wake up surrounded by panicked individuals with a severely swollen head and no clue as to what happened ... then you throw up.
Found this on Quora:
A bunch of studies of head injuries in primates were done at U Penn some years ago. Bottom line: it took 3 times less energy to knock an individual out with a ROTATIONAL blow (e.g. roundhouse punch) than with a TRANSLATIONAL strike (e.g. a jab).
There are 2 main mechanisms:
1) Cerebrospinal fluid (CSF) can cushion a linear blow, but not a
rotational one (the CSF and brain spin side by side in a rotational
2) the upper brainstem and higher brain can twist on each other
because of their size, shape and relative positions (and the arteries
that supply them can also twist), kind of like an apple and its stem,
causing loss of function precisely where "consciousness" resides in
the brain (the reticular activating system)
The studies mentioned by the Quora poster might be any of the many studies that came out of the Head Injury Laboratory at the University of Pennsylvania, but is most likely referring (primarily) to "Physical model simulations of brain injury in the primate", by Margulies SS, Thibault LE, and Gennarelli TA (published in the Journal of Biomechanics in 1990).