I would like to say the blood issue or shunning or their non-action/non-reporting of crimes because of the two-witness rule. I would like to say one of those because they have done so much harm to so many victims.
But the greatest harm has come from their ever-changing "generation." All JW's and those who care about JW's are held victim to a group that thinks the end is so ever imminent because of the "generation." For so many Witnesses, the end was coming when "generation" meant one thing. They string those members along now because they don't want to be faithful for so long and then lose faith right as "generation" changes but still may indicate the end is near.
You could also skip using any official definition of "generation" or any doctrine and just say the wackiest JW thing is that they have been and still are a Doomsday Cult.