Too many to pick one. And actually, I think a couple of really nasty ones that I've seen from ringside at lower level shows were the ones that really scared me. One guy (heavyweight) at an amateur kickboxing event in Norway around 1990-1991 took a beautiful right on his chin, got knocked completely cold and remained unresponsive for a long time. I was like 15-16 at the time, going HOOOOOOLY FUCK THIS MAN MIGHT DIE!!§!§" They carried him away, and he ended up ok, but from what I recall he hung 'em up right there and never fought again.
The other one was in Newcastle, Australia in the late 90s. Two heavyweights again, one of them walked straight into a huge right hand and looked dead before he hit the canvas, as if someone had shot him right through the brain with an assault rifle. And his leg wouldn't stop shaking. He lay there completely gone, fully asleep, with his left leg shaking / vibrating for longer than I had ever seen something like that go on. As if the last few volts of electricity were leaving his rapidly dying body. Later that night I had a chat with the ringside physician who just brushed it off. "Happens to rugby guys all the time, mate" or something like that. Made me grateful I wasn't into rugby.