What is the funniest insult / joke you've come up with on the spot?
For me it was "Your mom looks like Gimli from Lord of the Rings.". My best friend took a few seconds to process what I just said to him and then started crying laughing. Good times. I think it was the deadpan delivery that killed him.
A young woman I knew was complaining that she had made plans to meet a friend in the city (long before most people had mobile phones), but the friend hadn't showed up so she waited for a long time on the street corner.
My brother immediately chipped in, "what was his name, John?"