Think claws and fangs are the scariest part? Sometimes the fear's a lot quieter. The worst monster I've encountered in a book wasn't the one that roared; it was the one that watched. That creeping, patient, and ancient intelligence that doesn't just want to consume you physically but seems to absorb your very sense of self. They don't just kill you, they unmake you, showing you how insignificant your life is in the face of something so utterly alien and old.
I'd point to the Weavers in M. John Harrison's 'Viriconium' sequence. They're not described in gory detail, but their ability to warp reality and history around them, their incomprehensible motives, and the sheer existential dread they evoke is far worse than any jump scare. The fear isn't about being eaten; it's about your memories being rewritten, your world being frayed at the edges until nothing you know is true anymore. That's a much deeper, more lingering kind of horror.
Even in romance-adjacent stuff, the scariest 'monsters' often have a beautiful, captivating facade. The predatory charm that makes the victim complicit, the allure that masks the consumption. It's the fear of being willingly devoured.