I think Cthulhu and the entire pantheon of unearthly, utterly unimaginable deities that go along with it—precisely because they are so powerful, strange, unknowable, and dormant.
Godzilla will always be king of the Kaiju-type monsters—horrors that MIGHT have existed in a nuclear age earlier in Earth’s history that might be revived by nuclear bombs.
Freddy Krueger, slaughtering people in real life from the other side of sleep, will always be the ultimate boogieman in the shadows: impy, devilish, bully-like, and looking like the ultimate villian in the shadows and corners waiting for his opportunity.