After interacting with Jehovah Witnesses publicly in person over the last couple of months, I am always amused to see the shock and horror on their faces when I use their own New World Translation against them. Discussions with them usually go round in circles, and the best tactic that I have found is to demonstrate things from their own bible, not indirectly, but directly. Here's a specific example of what I mean, using the NWT (if anyone shows this to a JW, you will always get the same shocked reaction as I did):
Isaiah said these things because he saw his glory, and he spoke about him.
(John 12:41 NWT)
So, what's going on in this verse? Remember that JWs believe Jesus to be Michael. But, if it can be demonstrated that Jesus is Jehovah (using their sloppy rendition of the tetragrammaton), their entire theological worldview collapses like a house on sand. Thus, ask them to see if there is an Old Testament footnote next to John 12:41 in their bible. And sure enough, their NWT points to this passage in Isaiah:
In the year that King Uz·ziʹah died, I saw Jehovah sitting on a lofty and elevated throne, and the skirts of his robe filled the temple ... Then I heard the voice of Jehovah saying: “Whom shall I send, and who will go for us?” And I said: “Here I am! Send me!”
(Isaiah 6:1-8 NWT)
In the context of John 12, Jesus did miracles that John sees as fulfilled prophecy (in John 12:38-40, he quotes Isaiah 6:10 and Isaiah 53:1). Thus, in John's point of view, Jesus is the prophetic vision of Isaiah's writings, and John concludes (in v. 41) that when Isaiah wrote these things, he likewise "saw his glory" and "spoke about him." John is claiming that Isaiah saw the pre-incarnate Jesus. The NWT footnote goes directly to Isaiah 6:1, the reference where Isaiah says that he "saw Jehovah sitting."
Thus, according to John, Jesus is Jehovah in the NWT.