As the longtime friend of a crazy guy in high school who had one and liked to sneak up on people and use it (when they were napping, usually resulting in my beating his ass); in my experience, the worst spot to be zapped is the bottom of the feet.
I just didn’t know if a tazer with say, half a million volts applied to the forehead would permanently scramble a brain or if applied to the breastbone, could knock the heart out of whack. I figured if I had to use one on an attacker, that’s the two areas I’d aim for.