Not so much a one-liner, but one of the best put-downs I’ve ever come across on Usenet:
“I’m not calling you a hypocrite. I’m not sure you’ve the intellectual wattage for anything so advanced as hypocrisy. My theory is simply that you’re a f*****g idiot.”
This is a real one. I was grilling birdseye potato waffles in my Grans one evening, while I was taking them out of the packet she shook her head and said “its an insult to the spud”.