When I say brutally dethroned I mean knocked unconscious with a single strike.
Not JDS - Cain like where Cain was still seeing stars, but where the champ is seeing nothing.
This could be a no brainer, but I was just curious.
Chuck vs Rampage 
easymfe -Chuck vs Rampage
Thanks, but if I recall correctly (could be wrong), Chuck was still conscious and had to have a couple more punches rained down upon him.
Not a UFC fight but Pat Curran brutally dethroned Joe Warren. That was about as brutal a beating (and dodgy reffing) as I've seen in quite a while.
http://www.youtube.com/watch?v=rjAGtQosof0
Game
Set
Match
Shogun vs Machida? 
It has to be one punch? Can't think of any.
Your definition of "brutally dethroned" is very peculiarly specific.
Irule - AA vs Sylvia II? Can't remember if it was one strike.
One shot dropped him, followed by more shots on the ground.
By the very definition, Machida was brutally dethroned Shogun at UFC 113.
Don't know the last but Silva/Franklin had to be the most brutal.
Brutally Dethroned does not necessarily mean single strike, OP.
This thread is destined for failure.
I thought Serra taking GSP out was pretty brutal. After the big shot from Serra, he got some serious gnp on st. pierre.
XxLiveBaitxX - Not a UFC fight but Pat Curran brutally dethroned Joe Warren. That was about as brutal a beating (and dodgy reffing) as I've seen in quite a while.
http://www.youtube.com/watch?v=rjAGtQosof0
Game
Set
Match
That was a savage beating.
TheDecider - Don't know the last but Silva/Franklin had to be the most brutal.
This, silva came out of The depths of japan and the UK MMA scene, 1 fight in the UFC before literally obliterating rich, it was not even competitive, the others mentioned had back and forth moments really
The jones vs shogun bout was extremely brutal but doesn't fit OP's qualifications I guess.
I dont get it. Cain went totally limp and totally out for a split second.
Wasn't one punch, but Andy brutalized Franklin
Pretty sure Junior brutalized Cain, for you to deny that would be bizarre.
Um, maybe Jon Jones vs Shogun? Shogun was tough enough to take what Jon was giving, but in the end - it was just too much.
Chuck vs Randy II comes to mind as well, Randy was out - not sure if that was for the title though...don't remember.
GSP vs Matt Hughes II was pretty damn brutal.
And Machida vs Rashad as well, that was just fucked up.
Anderson vs Rich I was pretty bad, but Rich wasn't out completely.
Shogun Rua was nearly decapitated. Does that count?
Serra was pudding blood for about a month after gsp 2