One really good question to ask is about their 'Divine Name' brochure and their 'Kingdom Interlinear'.The Divine name brochure blames the fact that the 'name' of God is missing in all of the ancient Greek copies of the original manuscrips on 'apostate' christians who took over in the early 2nd century. The Kingdom Interlinear goes as far to say that these writings where 'tampered' with.
Fist ask a JW if they believe that the Bible is Gods word. They will without thinking say yes.
Then ask "So God has preserved us his word for our guidence without error". Again they will say yes.
This is when you present to the JW that their religion teaches that the word of God has been tampered with, and needed changed by the NWT translation committee.
Then ask, if what you feel is one of the most important features of the Bible could have been tampered with and removed in the 2nd century, how can the rest of the Bible be trusted. Couldn't these 'apostate' christians have changed whatever they want?
I would also suggest asking them exactly how these early christains where able to take the name out of the New Testament. Keep pressing the issue. The JW will eventually have to admit that the name Jehovah either isn't in the New Testament, or that the Bible can't be trusted at all. And don't let them get into that Septuagint nonsense, where talking about the 'conspiracy' they try and say happened. Make them proove that people where working to take the name out.