It may not be the best KO ever, but my personal favorite is Rich Franklin's ko of Nate Quarry. He was so knocked out that his entire body was stiff for a few moments, even down to his toes curling.
A few years back Charlie Kohler dropped someone cold in just a matter of a few seconds. I can't exactly remember who he fought, I just remember the impact. LIGHTS OUT!!!
Melvin Guillard vs Rick Davis. Davis got rocked and landed face first on the mat, might not be the best but it was a hell of a KO. I can still picture it in my head.
One of my faves. is Vitor's KO of Marvin Eastman that gash was brutal. And at one of the earlier UFCs when Gary Goodrigde knocked that guy out by putting him in a crucifix and elbowing him in the head.