Interesting subject.
It was exactly my experience with the Witnesses and starting to believe their teachings that got me to examine their beliefs and in so doing convince myself of the teachings of my own Catholic Church.
If you choose to try to evangelize (the the option is a polite “No, thank you”), I would go with the “seeds of doubt” option. The Witnesses’ organization is but a human organization (even they admit this); an organization led only by humans is not infallible, especially since it has made huge errors in the past, including predicting specific dates for the end of the world. I would point to the Witnesses’ Organization, then point to Jesus Christ and Scripture. If you can get someone to have an open mind and think for themselves instead of just believing the organization’s teachings, you might make progress.
A good place to start might be the issue of the Trinity, specifically the equality of Jesus Christ with God, which the Witnesses deny. Although He may not have used the phrase “I am God” or “I am equal to God,” He is described many times as having characteristics that belong to God alone (e.g., the Creator), and it would be easy to find something like this on the Internet.
Remember too that the Witnesses are very skilled in propaganda. Their routine is to persuade you to join their organization, and they are prepared with quotes from the Bible (often superficial) that seem to prove what they have to say. So, sincere discussion of the Bible on your part may not get you anywhere.
Good luck.