What takes the cake? Lemon Party.
Three old men having an orgy.
Or the rather famous "dildo split" incident.
Or the pain olympics.
Or that Russian or something guy that does... things to his penis. Like putting a screwdriver up it. Or breaking a jar in his ass.
Or guro, Japanese exhibitionism, based around the human body and ways to cut it up. While most of it's played for dark comedy, or as serious art that deconstructs human sexuality, there's that tiny, tiny fraction of it that's so horribly pornographic that it's just offensive to every single demographic that at all referenced in it.
Thankfully, I've never seen tub girl, blue waffle, or anything like that.
What takes the cake that's even bigger than Lemon Party, however... is either Sonichu or Encyclopedia Dramatica. One of which makes me laugh and vomit on a regular basis, though.
I suppose I do go looking for these things, out of a lack of empathy for humanity and morbid curiosity. And god, do I hate my curiosity.
Edit: Oh, how could I forget... haha, Rap Rat. It's a board game that VCR video. Unlike everything else here, I want you to look it up. It's not gory, it's not sexual, it's not emotophilic, or anything like that. Just look it up. It's scary as fuck, even though it should be normal, and seems like it would be normal to most people. It's the most horrific thing I've ever seen that isn't sexual.
I also once saw a prison rape on 4chan. It's not played for the sexyness of it or the fetishes that are surpsisingly common; nope, it's a real prison rape. Oh god, the blood, the crying...