Trainspotting: Renton's heroin withdrawal. He hallucinates that his friend's bloated and dead baby daughter crawls on the ceiling.
The movie doesn't do the scene justice. Besides just crawling on the ceiling, the baby starts to take on demonic traits and insults Renton about how he's responsible for her death.
Skagboys: Evocative of the above dead baby scene, in the prequel Nicksy is desperately rummaging around the bin at the bottom of the rubbish chute to save an abandoned puppy. Besides all the various detritus and gunk, he discovers what first seems to be a bizarre space alien doll. But much to his horror, he realizes that his one-time lover Marsha has done a DIY abortion job (very late along in the pregnancy) and dumped the kid among the garbage. The level of detail and chilling violence done to the poor foetus really hits home.
Filth: The scene when Costas is found: fingers and tongue cut off. Crucified with a nail gun. Eyes lying on piles of books before him, still connected by the optical nerves.
Marabou Stork Nightmares: Especially nightmarish are Roy's prolonged campaign of torture against the family dog (nine inch nails pounded through the jaw), and the gang rape scene.
Crime: Ray Lennox remembering being raped as a child.