I think the best way to challenge JWs when you have the opportunity is first of all to be aggressive.
Then ask them to show you proof from their bible (since they are bible students and read it every day) of these 3 basic teachings.
1. Show me proof of a 1st century Governing Body.
2. Show me how you see the year 1914 as an important year and identified in the bible.
3. Show me proof that there are overlapping generations.
Since NONE will be able to do this, and at best they will have to do backbends to even find something that might have some distant relation to it, it may get them to think.
Then you close with:
"Your entire religion hangs on these 3 basic beliefs, and you a bible student of X years cant find them in the bible? The only thing you can do is find a WT article that explains them for you?
Don't you think that's sad?"