It depends on what are the consequences to the person asking the question(s) as well as the emotional state of the person being asked the question(s). Previous posters have written good questions for the authentic persona of a JW to question WTBTS doctrine issues, but what question(s) will help a JW's authentic persona to overcome their cult persona? IMHO if one cannot peak a JW's curiousity and help their authentic persona to overcome their cult persona, seeds of doubt will not grow very quickly.
It is easy to determine when a question has stumped a JW because a JW will not be able to respond for minutes, so don't say another word until a JW says something.
I like simple questions that target a JW's authentic persona, like when a JW asks when are you coming back to the KH, ask "How are you feeling? (pause to learn if a JW wants to talk about how they feel) Why are you still a JW (with an empathetic and concerned look)?" Once a JW's authentic persona is in control, then start asking one or possible two questions something like "I was stumped by something that someone asked me. Will you help me?" and then ask the seed of doubt question(s).
Also, I like to learn how to respond to JWs when JWs use thought stopping platitudes. The nice thing about the WTBTS is that it teaches thought-stopping platitudes that one can learn how to overcome and then stump JWs when they use those thought-stopping platitudes. A good zinger may be able to pierce a JW's cult persona, because the JW is confident that they have the perfect protection. I hope that more people will share those good zingers for the thought-stopping platitudes, like in the thread Conversation-stopper STOPPERS needed!.
Peace be with you and everyone, who you love,
Robert