The worst tickling I witnessed was when I was a teenager.
There was a big man in his 40s in the neighbourhood with a thick beard, who enjoyed tricking a young girl, probably around 12 y.o., luring her to come close to him when he was sitting. She was always nervous, suspecting what might be coming for her, but probably couldn't say no to a grown-up man at her age.
So when she came close, he put her on his lap with her back to him, and started tickling her sides and ribs with no chance for her to escape his strong fingers. The big eyes of this blond-haired girl got even bigger when she felt fingers on her sides, and then the tickling began. She was just laughing helplessly and could do absolutely nothing to stop it, as her arms and legs were too weak from tickling, while her body tried to resist the overwhelming ticklish impulses.
I remember being terrified by the absolute lost of control that I saw. And that her uncontrolled laughter made all this look innocent and funny, so nobody near interfered.
That is why I think it was the worst tickling I witnessed in my life.
Even though it was just the tickling and in plain sight.