Oof, that's a heavy question. Unforgettable depends on what sticks to your ribs, you know? For sheer physical disgust and cosmic wrongness, nothing has ever topped the creature from 'The Troop' by Nick Cutter. It's a parasite, not some mythical beast, and the way it operates is so clinically, biologically vile. The horror is slow, internal, and utterly inescapable. You don't just fear it; you viscerally feel its violation.
On a completely different axis, the monster in Stephen King's 'Pet Sematary' is the grief itself. The Wendigo is the folklore dressing, but the real, unforgettable entity is that creeping, possessive sadness that warps Louis Creed's judgment. It's not a monster you see; it's one you become complicit with. That's a different kind of haunting—it lingers because you can imagine making those same terrible, loving choices.
Also, gotta throw a bone to the thing from 'Annihilation'. VanderMeer's 'Crawler' is unforgettable because it's literally incomprehensible. The book refuses to give you a clear picture, just these terrifying, beautiful, and wrong impressions—a tower that's a throat, writing on the wall made of living fungus. It's an idea-monster, and those are the hardest to shake because your own brain finishes the painting with its worst fears.
For pure, iconic design that launched a thousand nightmares, it's hard to argue with the xenomorph from 'Alien', even though the book adaptation came after the film. The perfect organism. But books let you sit in the silence with these things longer, and that's where 'The Troop' still wins for me.