The most devestating, and or the most one hit or sudden KO's? What's the perfect combination of power and speed?
Heavyweights score most knockouts. At heavyweight, a guy with a 80% KO ratio against tomato cans is nothing special (even for average punchers), while at lightweight it is a huge anomoly. Smaller guys often lack the power to put the other away. That's why fights at lower weight are more often grueling battles with more punch exchanges, cuts, bruises but not the concussive power that makes you hear birds.
Heavyweights are a given for sudden or devestating Knockouts. People forget about one divison though. Bantamweights seem to really get each other at times. I'm not sure why but this divison seems to be loaded with top level pound for pound punchers.