In John 20:28, Thomas sees Jesus risen from the dead, and proclaims that Jesus is his Lord and his God. Thomas was a good Jew, and certainly a staunch monotheist. He would never admit that someone besides the LORD was God.
This is a good verse to show Jehovah’s Witnesses. I’ve heard from various sources that they often have never seen this verse. When they see it, their immediate reaction is to say that Thomas was taking the Lord’s name in vain. Supposedly they think this is what Thomas was doing because that is their reaction when they see the verse. I’ve never seen any Jehovah’s Witness do this in person until the other day. In fact, I have it in writing for everyone to see. You can check it out here. The person who said it has made it clear that he or she isn’t a Jehovah’s Witness, but shares many of the same beliefs.