Well since I've done a good job of avoiding 2 girls 1 cup, probably goatse or tubgirl.
Just off the top of my head.
Oh wait, with those you can detach yourself afterwards.
Night Shift Nurses
Not the '80s porno flick, but the big-budget, remarkably well-constructed and utterly, utterly abhorrent 2000 anime series.
It's like 2 girls 1 cup, goatse and the worst of 4chan all rolled into 10 episodes of immaculately produced filth. Not smut. FILTH.
I gave up after the first episode, and that was before we even got to the rape, the blood-play or the scat.
Oh and the girls aren't doing it because they want to, either (at least not until later when they're all broken shells of their former selves). They're mostly blackmailed, manipulated or outright raped. But they're given enough depth for you to feel pity for them.
But why am I writing this? I should just give you this link and tell you to read the user reviews of the poor saps who watched it the whole way through. They convinced me that I had the right idea in running away after ep 1.
http://myanimelist.net/anime/1387/Night_Shift_Nurses
Failing that, Ninja Ressurection. It's much shorter and it doesn't scar you quite so permanently, but it does have an underaged girl getting raped to death by a very large demon.
Actually you know what, that does scar you permanently. It's bad shit.
Go watch Card Captors or something.