I think Rahman vs. Lewis might be unparalleled in terms of devastating finishing blows landed in the same rivalry. Schmeling vs. Louis I & II, maybe?
Donaire on Montiel. Pacquiao on Hatton. JMM on Pacquiao. Pretty much all of Tyson's KO's. Mercer on Morrison Williams on Jack. (damn this was brutal) EDIT: This should be on the classic forum.
That first knockdown punch Matthysse Landed on Peterson was nice. The Hatton/pac fight knockout was also nice..
Tua vs Ruiz 19 second knockout. Which probably led to Ruiz clinching so much further on in his career.
The Lewis uppercut vs Vitali was the biggest punch i have ever seen even tho it wasn't a ko(would of been if it was any other fighter)