In the UFC, Irvins flying knee to Terry was awesome, but I can't imagine a better KO than BJ Penn knocking Caol Uno cold in their first fight. Uno's face when the ref pulled Penn off him was priceless & Penn's reaction after was perfect. Leave the locker room, have a fight & return to the locker in less than five minutes.
The other fighters in the locker-room must have thought he's forgotten something!
In Pride I'd have to say Liddel Vs Mezger was one of the best I've seen. Again purely because of the way Mezger crumbled.