That's a tough question for me because I don't often times watch KO's unless it's a video of highlights. Probably sounds kind of weird but I can't just watch knock outs cause then I'll feel compelled to watch the entire fight. To me, it's like watching the ending of the film only.
Hearns/Duran. I never watch a ko for the brutality or joy, I think thats moronic, but rather to observe the technique.
likewise. that faint flicker double jab at the chest, the right cross to the chin, bang.
The ones Ive seen the most I can remember are
Martinez v Paul Williams 2
Pacquiao v Hatton
Mathysse v Peterson
I can watch them alot because they end within 3 rounds, so i can watch the fight from start to finiish.
Not a clean KO, but probably Tszyu cementing Judah with that right hand. Judah's break dancing, followed by his tantrum after being stopped is pure comedy.