MPD Psycho/Nightmare Fuel


 * There is at least one instance per chapter (there are 14 volumes (9 in english), but it drops off for psychological trauma in later chapters), but one of the stand-out moments of horror for the series was when the protagonists investigate a case where a serial killer was using the brains of his victims like grotesque flower pots. Made even better by the fact that up until he buried them, the victims were still alive while the flowers were growing.
 * How about the First Chapter, where the main characters girlfriend is mailed to him in a styrofoam box missing all her arms and legs. Oh, and she's still alive.