The Wasp Factory by Iain Banks. The narrator and main character is a psychotic teenager, and being inside their head just feels so gross. Fantastic book, but genuinely disturbing.

In close second is Earthlings by Sayaka Murata. The main character goes through some stuff as a child, and comes to believe that she isn’t human. Meets some others like herself and it gets weird. Great book, not for the faint of heart!