I just heard this one on a rebroadcast of the old BBC quiz show, “My Music”.
The German conductor Hans Knappersbusch once conducted a mediocre orchestra. After the performance, the orchestra manager asked him, “When was the last time you conducted our orchestra?” His reply: “Tonight.”
OUCH! 
So post the best, shortest, and most pointed put-downs you’ve heard of (or perpetrated yourself).
Garrison Keillor’s Guy Noir once told a tough-guy, “It’s a good thing your mother isn’t here. She’d be very disappointed in you.”
I once told a guy he was a terrible waste of skin and hair.
–Nott
My dad says, “You could be replaced by a high form of plantlife.”
Something I said in my wild youth:
Random man: Hey, baby, where’s the party?
Me: Your house. Go home.
From Smokey and the Bandit:
Admittedly, it’s only good for putting down your own child, but still. 
Another favorite:
My cousin as said out of his car window to another who’s female driver just cut him off:
“I hope you die of a long, painful, expensive bout of ovarian cancer”.
The “expensive” part really got me.
A group of teenage boys driving past a pretty girl. One of them shouts, “Hey baby, do you fuck?”. She quickly responds, “No. But my brother does. Bring your mother around sometime!”.
You’re so low that you have to look down to see up!
(Yes, I’m six years old.)
“One” must look down "One"s nose for this one.
Regardless of the question…if it is impertinent, rude, uncalled for, intrusive…the answer is always this one.
“ExCUSE me?”
[sub]Gotta have supercilious attitude, but…[/sub]
This WORKS…trust me !
From my mother, whenever any of us began whining instead of trying:
"Think of it as an intelligence test."
John McKay, head coach of the Tampa Bay Bucs, was asked after another loss:
Reporter: “How do you feel about your team’s execution?”
McKay: “I’m in favor of it.”
I mean, of course, that you have to look up to see down. :o
Oh dear. I think someone suffers from delusions of relevance…
Sorry, I’m on a dreadful hotel browser, that was meant to appear in quotes. I’ve always just threatened to use that one, never actually had the chance.
What do you think of Western civilisation?
I think it would be a good idea.
–Gandhi
I’ve been called worst by better people.
When someone asked to speak to my “superior”: I have no superiors and damn few equals.
When someone makes a really rude and uncalled for remark, I just say “AND?..”
…one time, for some reason, my ex invited me and some mutual 'net buddies into a chatroom. This was after we’d broken up. He started talking about being extremely…ah…frisky, and wanting to visit a, shall we say, working girl.
Him: “Hey, anyone wanna lend me $40 so I can see a whore?”
Me: “Sure, I’ll do it. . .(new line). . .you need the practice.”
That got out just about all of my anger towards him…'specially sine everyone laughed.
I have occasionally referred to someone as a waste of carbon.
I can’t remember the names, but it’s a telegram conversation between author and publisher:
A - Must have quick answer as have other irons in fire.
P - Suggest you remove irons and insert typescript.
And still in literary vein:
Dear sir, I am in the smallest room in my house and your MS is in front of me. Soon, however, it will be behind me.
After once having a woman berate him for ‘inappropriate’ behaviour while drunk, Winston Churchill is alledged to have replied “Madame, in the morning I will be sober - you will still be ugly”