My favourite question is:
What would convince you that the WTS didn't have "the truth"?
The reason I like it is it forces them to consider the possibility, which is a big step in itself, and it also may elicit a response that turns out to be something they have done!
If the reply is "nothing, it can never be so!!" then "wow, that's what someone in a cult would say!" may make them think.
It's different to just saying "you're in a cult", because you've reasoned them into admitting it through their refusal to even consider that they could ever be wrong. Forcing them to say it out loud means they hear themselves saying it. Makes it more "real" IMO.