Been thinking a lot about this stuff lately and since I'm just in the process of making some new ko clips out of some popular series I have noted that i enjoy the part where the heroine goes under the most. The actual part where she looses consciousness, her eyes slowly close, her arms start to go limp, her knees bend as she falls and eventually her back arches as she goes out witness a final *uhhhhh* that's the best part for me. Everything else is just an added bonus for me.
Second to that, I just enjoy every shot or panel that illustrates their limpness.
I agree with you that that is the best bit, but i also like it when there is a scene showing the badguy about to strike.I also like it on head ko's if the badguy makes a witty comment just before, or as he hits her.
I think the reason we like to see the villain talk to the superheroine while she is unconscious is because he can taunt her or insult her and she can't respond. She can't say anything back to him because she is out cold and doesn't even hear what is being said. Yes that is HOT.