What are the darkest, hardest-hitting and most brilliant novels you’ve ever read?

I’m looking for novels with a truly dark atmosphere, morally ambiguous characters, and uncompromising storytelling. Think High Life by Matthew Stokoe or American Psycho by Bret Easton Ellis — literary, intense, and at times disturbing. Not your average crime thrillers — I want the raw, the gritty, the psychologically complex. Bonus points for themes like drugs, antiheroes, existential dread or sharp social critique.