Easy - Galatians 1: 8-9
I agree Sea Breeze that there are probably many scriptures one can use to demonstrate Watchtower doesn't have the truth as taught in the Bible. A good one is reference Galatians to demonstrate they do not teach what the apostles taught, as you say.
But I think there's an even simpler way.The Jews proved their faith by abiding by the law. But we were always taught a Christian now has their conscience to use instead (combined with the Holy Spirit).
Yet over and over exJWs will say they felt there was something 'not quite right' about the teachings and practices, behaviour and harsh treatments meted out by watchtower. That was our conscience reaching out to us .......but witnesses are forbidden to abide by their own consciences. That, to me, is what really marks out Watchtower as a cult