"If I were a criminal wanting to hurt you, what could you do about it?"
That, and variations of it, have been by far the most effective...
I like to throw in the "If I were the bad guy you accuse me of being, what could you do about it?" It debunks their backwards/presumptive insults, and how helpless they are at the same time...
Most of the anti's approach with something akin to "does your father know you're gay?" It is a loaded question that includes a false statement. Any response at all results in an admission of a falsehood. Well, unless you
are gay... You get the idea. These sorts deserve the ugly responses.
I still like the women that tell me I have a small penis for carrying a gun:
Bitch on Wheels: "Carrying a gun is compensating for a..."
Me: "The barrel is 5 x 0.5 inches. If that's compensating, I've got a surprise for you (wink)."
or/and
Me: "Care to test the theory?"
or/and
Me: "There's a gas station bathroom right over there, I'm sure you're familiar with it..."
Then you wait about 10-30 seconds for her to realize what that means... Just one more reason to carry a recorder...