Which is the deadliest punch in Tae Kwon Do?

Answer 1

That's hard to say - some instructors would say the deadliest punch is the one that takes out an opponent more or less permanently, so anyway you accomplish that ...

Punches that hit a softer or more exposed part of the anatomy are very deadly, and usually don't require much power to cause severe damage. For example, a punch or strike to the throat, to the eyeballs, nose, etc.