What is the biggest sexual scandal to hit the top of JW tree ?
With a closer reading of the title, focusing in the "top of JW tree"; speaking just from reading about some of it:
Old:
Rutherford Russell and their adventures.
Modern:
Barbara Anderson's revelations about child abuse.
Greenless and Chitty within the GB.
Bethel wife swapping the 1950s.
I am sure that there are many more.