The entire "Night of Terror" sequence, as it's called on the score, which is basically 10 minutes of Nina going completely MENTAL the night before her performance. Unsurprisingly, that is director Darren Aronofsky's favorite part of the movie.
The major turning point is Beth (Winona Ryder) stabbing herself in the face with a nail file whilst screaming "I'm nothing! Nothing!" And then her face turns into Nina's, who runs for her life as Swan Lake music, the introduction to Swan Lake, BLARES over the soundtrack. And when she gets to the elevator, she discovers she is holding the nail file. All of the other hallucinations reveal themselves to be hoaxes shortly afterwards, but this one is so extreme, it fully confirms that we have reached the point where we can't trust ANYTHING about the movie. And consider that it may have actually been Nina who stabbed Beth.
When Nina gets home, someone whispers "sweet girl" from the darkness of the kitchen and there's a bloody figure standing in her kitchen when she turns the light on. And then the paintings start screaming at her.
Listen to the music from that scene and you'll probably never be able to listen to the introduction of Swan Lake again without hearing weird sounds in the background of the music.
Nina's final Black Swan wardrobe and makeup is more than a little frightening.
Her first appearance as the Black Swan, including with Nina's initial lunge to the camera, with that disturbing growl.
Then there's the moment when, after noticing the blood on her fingers, she starts cutting her nails whilst looking in the mirror with a psychotic expression on her face, while disturbing laughter can be heard...
The finger damage. The finger damage. Not much tends to squick me, but I had to look away from the screen when that happened.
The part where Nina slammed the door with her mother's hand still on the doorframe. Her mother's screaming in pain only makes the scene even worse.
And then shortly afterwards when she twists her mother's injured hands. Did this movie have a thing against hands or what?
The climax (pun not intended) of the sex scene. Lily stands up and whispers "Sweet girl" before suddenly transforming into Nina's doppelgänger and attempting to smother Nina with a pillow. The way sweet girl is said.
The things that are obviously unreal are disturbing, of course...but the things that might be real are even more unsettling because by then you realize you can't trust anything about the movie.