Devil May Cry 3's Boss Rematch room, the Lost Soul's Nirvana. This entire room is an inescapable MC Escher Theory of Relativity room of staircases colored and shaded only in tones of light gray, white, and dark gray without definite shadow, and all littered throughout are big enjeweled monoliths of various bosses and battles you've faced throughout the game. When you do face them, their arenas are also all drabbed out, and their music is slowed down and distorted to provide a sense of offputting dissonance meant to mentally throw you off. A true mellowed nightmare endurance test.
Devil May Cry's Mirror World. Everything in this world is backwards, the textures bleed a heat haze like ooze, and the most unsettlingly creepy music plays while you explore for the Philosopher's Stone.
I also like how older RPG dungeons have a dissonant dream like feeling to them.