Devastating insults?

[Ash]

"You are a smelly pirate hooker!" 
Why don't you go back to your home on Whore Island!?"

"Your hair...looks...stupid!"

[Trevor]

Jack Watson in The Wild Geese as the RSM had a few good ones:

To Roger Moore who has just taken a heavy fall: "That was ludicrous, sir. You are jumping from an aeroplane, not a whorehouse window." 

To his men: "Some of you know me already. Those of you who don't are in for a big, big f*****g surprise." 

To Richard Harris: "Lt. Janders sir, if you do not get up and run, I will personally string you up by the balls, do I make myself understood?" 

To a prone mercenary: "On your feet, you f******g abortion!"

To the medic: "You screaming faggot! Move it before I slide up your arsehole!" 

[Jack]

An old favorite from Saturday Night Live:  "Jane, you ignorant slut..."

[Dennis]

Remo Williams - "You know Chiun, you're a real pain in the a**."
Chiun            - "That is because it is the fastest way to your brain."