As the subject says, what is the most graphic or, rather, disturbing picture, creature, enemy, or video you have ever seen, something that as you look at it consumes you in wide-eyed open-mouthed horror.
This, well, thing is the single most horrifying thing I have ever seen and probably will ever see.
Although a close runner up was this, erm, thing.
I couldn't trust the underside of my bed for days after watching this. *shudder*