I like to ask them how did Jesus inform Rutherford that the Bible Students would be his exclusive channel on earth in 1919?
To date I have not gotten even an attempt to address this point.
I ask "If I claimed that in 1985, I was selected by Jesus to exclusively preach his will, what would you say?"
Invariably they would scoff and ask what proof I had.
Then I follow up with the question above. You can watch them starting to shut down trying to explain why they would demand proof for my claim but not of their leaders. They cannot refute my claim without refuting the claim of their Governing Body.
As a side note, there actually is an answer. In "The Finished Mystery" Rutherford claims that Jesus sent C. T. Russell as an angel to continue to guide and direct the Watchtower Corporation. I don't usually bring that up since Witnesses seem trained to dismiss much of what was previously written as "old light".