The best response to "fuck you!" is to say "Don't threaten me with a good time" with a suggestive look on your face, amirite?

Unless the insulter is one of your siblings...

It's good but i prefer "Not before you take me out to dinner" with the suggestive look added on of course.

I usually respond "Yes please."

Unless you wanted to insult that person, then you'd say: "You shouldn't offer things you can't deliver".

'Maybe later' or 'not today, dammit, get your mind out the gutter!' Are two of my favourites.

Not the "best" response, but definitely high on the totem pole of sardonic comebacks.