Given that Jehovah’s Witnesses have heard John 1:1 quoted to them so many times they can, quite literally, respond to it while in a comatose state, I suggest the memorization (yes, memorization–the real thing, not the “can’t I just stick a little page flag thingy in my pocket New Testament” thing) of a passage that Jehovah’s Witnesses do not encounter on a regular basis, Colossians 2:9. Now, of course, memorizing one passage of Scripture will not prepare you to do battle with Jehovah’s Witnesses. I’m talking about those “fast” opportunities that you may have on a train or a bus or in an airport where you want to have a passage at the ready. Colossians 2:9 is not one the JW’s hear with regularity. Of course, you don’t just want to quote a verse. Here’s a possible scenario.
JW: Yes, I talk to many who believe in the Trinity, but I have never seen any sound defense of the belief from the Bible.
You: I wish we had time to talk more about it, but as I understand it, you believe Jesus is Michael the Archangel, while we believe Jesus has eternally been God, the Son. Likewise, you do not believe Jesus rose from the dead with a physical body. Yet, when Paul was refuting the errors creeping into the church at Colossae, he wrote, “For in Him all the fullness of Deity dwells in bodily form.” Note that not only does Paul affirm that the fullness of Deity dwells in Christ, a term that does not mean merely a “divine quality” but instead, “that which makes God, God,” but it dwells in Christ in bodily form. Since Paul wrote these words after the resurrection, it is clear he did not believe Jesus had been resurrected in a merely spiritual form.
Now keep in mind, the NWT mistranslates the key word in the text, theotetos, deity, as “divine quality.” I am unaware of any lexical source that provides such a rendering. Instead, as Richard Trench noted long ago while contrasting theiotes with theotes,
But in the second passage (Col. 2:9) St. Paul is declaring that in the Son there dwells all the fulness of absolute Godhead; they were no mere rays of divine glory which gilded Him, lighting up his person for a season and with a splendour not his own; but He was, and is, absolute and perfect God; and the Apostle uses qeo,thj to express this essential and personal Godhead of the Son; (Trench, R. C. (2003). Synonyms of the New Testament. (9th ed., improved.) (Page 8). Bellingham, WA: Logos Research Systems, Inc.)