Mmmm, quite so. In that case I'd have to choose the Weeping Angels or The Silence because can't see (or remember) them unless you look right at them. You know what they say "The most terrifying monster is the one you cannot see." I also have another choice if it's allowed, I'd say the Time Lords from the end of the David Tennant era. Think about it, they manipulated the master throughout his entire life just so they could resurrect a dead dynasty.
As for Davros being a human horror, I think of him as a monster of the mind, the dark, evil portions of human mentality that no one wants to acknowledge but we all must deal with its existence and it eats away at you knowing what your race is capable of.