my all time favorite (not just horror of my fav books in general) is "Exquisite Corpse" by Poppy Z Brite
I love love LOVE anything that mixes horror and romance. not in the "Haunting Adeleine" way (jesus lord EUGH atrocious for so many reasons), rather in the "Interview with the Vampire" way. anything where two horrible characters are in love so devoutly in an equally hideous way, and cling to each other life prey to bait, oh my LORD it blows me away.
this book in particular is about two serial killers (Andrew, a necrophile, and Jay, a cannibal) and analyses them separately before they meet in roughly the last third of the book, and the way one of them is fundamentally afraid of being alone, while the other of not being alone. I adore how even in the most mundane descriptions, the feeling of a filthy and somehow also sexual undertone never leaves, it paints SUCH a visceral picture that you can honestly feel it sticking to your gut. the power dynamics are delicious, side characters flawed and humanly fleshed out, emotional relations absolutely divine
very heavy-handed horror, some parts border on nauseating, and the way it mixes atrocity with poetry oh my god I could talk about it forever