Some good ones that come to mind whenever these threads pop up:
"I want to fuck chuck" -Wandy
"I promise violence" -Wandy
"Right leg hospital, left leg cemetary" -CroCop
"I mean, who else could box you up, take you down and gogo you? Fuck, put me in a magazine." -Nick Diaz
"Is that his nutsack popping out?" -Rampage
"Forrest is the kind of guy that when he wipes his ass, he doesn't get it all" -Rampage
"Damn I'm ugly" -Forrest Griffin
"I'm gonna knock Tito's hair black" -Ken Shamrock

And my personal favorite, "This is where I break his liver" -Bas Rutten
What are your favorite MMA quotes?