I think opposition has to be taken in to account so, Sugar Ray Robinson Vs Gene Fullmer II I think it has been described as the perfect punch, the timing is ridiculous. If you haven't seen it go on YouTube and have a look. Modern day I would probably go for Hearns Vs Duran, it is a shot that still makes me wince every time I see it.
Hearns against Duran Pacquiao against Hatton Martinez against Williams are three off the top of my head I'd have above it but it was a hell of a counter. In terms of significance KO6 is definitely up there though.
froch vs groves II, froch has kept quiet about it too, not mentioned it in every interview or anything like that.
This X100. The speed and accuracy was impeccable - literally perfect technique. He generated so much torque while hardly even drawing back, it looked as if he just flicked his wrist.