I am pretty good thinking on my feet so it is rare I don’t have the perfect comeback at the time. If I don’t it is unlikely I ever will think of it myself, though occasionally pick one up from other conversations or characters on the goggle box.
Some of my favourites:
“You are a classic example of the inverse ratio of the size of the mouth and the size of the brain.”
“For such a gatherer of facts here is an important one: You sir/madam are a nitwit.”
Or as I wrote in a review of the Central City Jazzmen : “I enjoyed the gig, particularly that there is nothing in your act which will bring 10 pin bowling into disrepute.”