By this point, we all know how to deal with zombies: aim for the head. Destroy the brain and you've put a zombie down for good. A hit anywhere else just wastes ammunition.
But what about reanimated skeletons? You're far more likely to encounter them than zombies, which are just imaginary. Aiming for center mass or the head will probably accomplish nothing. Brian J. Noggle argues that your best bet is to shoot the skeleton's pelvis:
Shoot a little high, you might be able to sever that spinal cord. A little to each side and low, maybe you’ll put the skeleton on the ground and incapacitated. A direct hit will probably do both, leaving the skeleton to only crawl toward you, allowing you time to escape. Or, I suppose, it could ride C3P0-on-Chewbacca style on another skeleton. Regardless, it’s one less coming after you under its own unholy power.
P.S. Brian wrote the novel John Donnelly's Gold, an excellent heist story set in a tech company.