What is your favorite KO of all time? Mine would be machida vs rashad. And the memes that followed were gold. 
I got a few that rank equal to me.
Fedor KOing Arlovski was pretty badass. Even though I love Arlovski.
Bigfoot KOing Overeem
Roy Nelson KOing Schaub
There's more but I can't remember right now.
The powerbomb was always my favorite pro wrestling move, so has to be Rampage-Arona. I didn't stop thinking about that one for weeks
Daley/Smith
or
Hendo/Bisping
also even though I loved Chuck, there has never been a more vicious punch landed than the one that Rashad landed on him. When Joe said "right...on...the....button..." it gave me the feeling maybe it was the hardest punch he had ever seen as well
Vovchanchyn - Bueno, or Alex Emelianenko - James Thompson.
Machida KOing Evans was the first I thought of, but I also really enjoyed Bigfoot clobbering Reem.
Shogun KOing Tehuna is up there for me as well. 
I love machida versus randy. That looked like some matrix shit
Badr Hari vs Stefan Leko via Spinning Back Kick
CroCop kicking Igor's head off was also pretty special.
whatislife - I got a few that rank equal to me.Same here with Fedor vs Arlovski!
Fedor KOing Arlovski was pretty badass. Even though I love Arlovski.
Bigfoot KOing Overeem
Roy Nelson KOing Schaub
There's more but I can't remember right now.
Also like Lawler vs Villasenor
And Fedor vs Zulu for the way Fedor fought him
Wand v Rampage II
RalphHarris -Yes. By far.
Hendo vs bisping
hard to beat the hendo slam on bisping (without trying to be cool and go way old school with it).... that shit was epic.
Condit/hardy
hunto-struve
Scott Smith KO'ing Pete Sell was epic
Hendo/Bisping - Bisping had it coming.
Evans/Liddell - I love Chuck, but I just remember thinking how violent that punch was when it happened.
Anderson's uppercut elbow against Fryklund. Not sure anything similair has happened before or since. If it has please link it!
RalphHarris -Hendo vs bisping
Or Weidman V Silva 1, the guy had it coming.