...in the context of a relationship breakdown..?
mine was, finding out my ex had been screwing around behind my back, having a huge fight and his parting shot was, 'no man will ever want you now! i mean, single parent - three kids!' 
I'm pretty sure he isn't the biggest c*nt out there! :) (and i do know it wasn't true!) But i really think it's cathartic to write it down...
alternatively, what was the worst thing you ever said? ( I of course, am a paragon of virtue and have never said anything mean or cheap!)