There is a series called Fight Science that delved into this exact debate. During the first episode they checked the power of different men training in boxing, wushu, karate and taekwondo. This episode fell under scrutiny due to the fact that not all the men were of the same weight class. Be that as it may, the boxers tended to generate the most power due to the technique of them using their whole body weight, rather than the Asian martial arts that focus more on precision. Mixed Martial Artists entered the bunch, and were about on par with the boxers.
All this being said, if the goal is to knock your opponent out, what matters more than punching power is punching accuracy. If you hit a fighter on “the button”, or just under the chin, the human body reacts to the shock of the punch much worse.
All-in-all, technique surely makes a KO punch more powerful, but what determines power is much more complex than that. The reader must remember that different bodies react to strikes differently. Some fighters have stronger bone density, which lessens their chance at breaking their ribs from a kick, while others bodies have a higher KO threshold to their brains. Similar to different fighter pilots blacking out a different G-forces, the same is true with fighters getting knocked out.