Totally depends on the situation. You use something that the situation presents to you. It probably can’t be planned in advance. My brother once told me that I had a knack for saying “fuck you” in such a way that a person would walk away saying “thank you,” and not realize until long afterward what I’d said. I thought that was a great compliment even though I was never sure if it was true. Anyway, you’re right, it’s best done in the politest of language. Dear Abby always knew how.