Hmmm. Three-way tie, TBH. I've been on the internet since before Netscape, so take this for what it's worth.
A child (maybe six?) being raped was definitely one of the worst; she was screaming and crying in the brief clip I watched. Apparently it was her father that was raping her; AFAIK he's in prison for life now. Hopefully his life in prison is very short. I'm not even sure how many years ago this was now.
Crush videos involving kittens on an .onion site about a decade ago. I clicked on a thumbnail just long enough to verify that yeah, it was what I thought it was, and then left the site. I've owned cats my whole life; if I knew someone personally that did that, I would absolutely kill them. Yes, I had a stronger visceral reaction to that than I did a child rape video, and no, that doesn't make any sense.
A video from Mexico showing a purported child rapist; he was restrained, and dogs were chewing his genitals off. He had those high, breathy screams of someone in agony that was unable to catch their breath. It ended after the dogs tore his entire lower abdomen to shreds and one of the cartel (?) members cut his throat. Maybe 15 years ago? I'm pretty sure that this was a hi-def video.
I didn't watch the video of the Russian officer cutting the head off a Ukrainian POW. I'm past the point of having any desire to watch videos that further decrease my opinion of humanity.