We were off to a good start but got sidetracked on some doctrinal issues. Pirata, thanks for starting that new thread.
I'm looking at your responses and thinking of how I could use them. Many of the questions mentioned are ones that I myself had (proof that the WTS is God's org, issues with a global flood, extrapolation of Jesus words from the past to modern day). While these are interesting, do they get deep enough to really get the wheels turning? Do they plant seeds (good analysis, whataburger)? Remember the type of mindset that we're dealing with.
I also fully recognize that there is no silver bullet.
Pirata said, "I think the only effective question is one that matches their existing dissapointment or issue OR is something that they have never heard before and will want to research." Agreed. That means we have to be good listeners so as to recognize an opportunity when one arises.
A couple of you described questions that could lead to further questions. That's good. Start simple and broad, and let the conversation take a natural course. Allow the other person to choose their answers and then lead them to the next logical and rational question.
IMHO, the most effective question I have seen so far was one posed in Butterfly's post above. "What makes our set of beliefs more right than everyone elses?" The reason I like this is because it is broad enough to open the door to a number of follow up questions based on the response of the answerer. You cold pose this question in a very unassuming way. Unlike starting with a specific doctrinal issue that may raise a red flag about some "problem" you have with WT teaching, or that could quickly be stonewalled by a clever twist of JW logic, this question has a more universal, open minded sound to it. A possible scenario starter:
FM: I was informal witnessing (nice, right?) to a Muslim guy at work last week and he said that our religion had some really nice similarities to Islam. He was raised Muslim and is very devout. Any question I asked him, he could answer with confidence. It got me thinking, are our religions really that different on a fundamental level? I mean, how do we really know that we are right and they are wrong?
From here, it could go in many different directions. For instance:
A: We have what they don't have in the faithful and discreet slave. Jesus said in Matthew that they would be the ones to provide our spiritual food at the proper time. They have been doing that since 1914.
FM: The thing I don't quite get is, why do some of truths change? The JWs of the 1914 era are really different than us today. There's no way we would teach (insert wild doctrine, faulty prophecy, bold claim) today. We know that's wrong.
A: ...light gets brighter...
FM: That's true sometimes. But Jesus said, "food at the proper time." Do you think that he approved of, or even directed the slave in, dispensing innacurate teachings? How could teachings like (take your pick from the buffet of wackiness) be considered the type of thing that Jehovah and Jesus would be pleased with? I have had a little trouble figuring that out.
A: Imperfect men sometimes make mistakes in the way they understand things. We need to focus on the good things that we know to be true now.
FM: Agreed. And I know the early JWs meant well and were very zealous. They thought they had the truth about those things then. It makes me wonder though, what don't we know now? I think about that guy I work with. He means well and is very zealous. His religious leaders have made mistakes too but there are many moral and ethical teachings in Islam that we would consider to be true. If we just focused on those good things, then could the teachings of Islam be considered "truth?" I just feel like like I'm being unfair pointing out the things that I think are wrong about other religions, then writing them off as "untrue" when we have had our fair share of mistakes too. That's why I wonder sometimes, how do we really know that we have the truth?