I think its probably Hearns/Duran too, the KO could stand on its own as the "best" but then you throw in who the two fighters were and that adds something to it.

This is another one of those similar to Hearns/Duran in the viciousness of the KO combined with how great the two fighters were

http://www.youtube.com/watch?v=9UpzS6kfSNU