"Actually, that's really rude."
For all those occasions in life (and from reading the AIBU threads, it seems i'm not the only one who gets them) when someone says something so rude that you just don't know what to say.
We all operate on the principle that other people will be as polite and reasonable as we are (or wish we were!) so when someone is really rude, we just don't know what to say and end up spending hours later wishing we'd said something.
So practice this one line and be prepared, so the next time someone tells you you're fat, or your baby is ugly, or that you're just lazy, repeat after me... "Actually, that's really rude."
Add your own useful phrases here; I'll start with "I'm not going to discuss that with you."