I think this is something where the ex-JW community just gets way too technical. For instance if someone were to ask about the 1914 doctrine within a minute someone who isn't familiar with the teaching is going to be lost.
For a "Silver-Bullet" question I believe you're going to have to paint in broad strokes. If it were me I would ask one simple question that leads no where:
Please show me absolute, positive, 100% irrefutable proof that what you teach is the truth without a shadow of a doubt?
What are the answers preaching, more moral people, 144000, interprets the bible correctly, there is nothing that is 100% irrefutable proof.
Preaching - Many other religions do that, what does that have to do with having the truth.
More Moral People - Please show me statistical proof of that.
144,000 - How do you know positively that there is a limited number of people in heaven, have you been to heaven? Did God tell you? How do you KNOW your interpretation is right?
Interpretation of Scriptures - This religion has 3 scriptures backing their belief, this one has 5, could you please show me irrefutable proof that your interpretation is the right one?
I think that one question only leads to dead ends. The thing about the question is that every JW will try to answer it when asked. I can't see it having an answer, because nothing is 100% irrefutable. But bless those JWs for thinking what they have is easily proven as being irrefutable.