Ready When You Are Sergeant Pembury.
I haven't quite decided if it's Thomas Harris's writing, or Anthony Hopkins acting, which makes Hannibal Lecter, possibly the scariest movie villain of all time: it's probably a combination of both. Though the success of the character is really what makes it scary. This is a certified multiple murderer, who has no qualms about skinning a man's face to use as a mask, or killing a musician for playing badly.
And yet at the end of the film somehow we are rooting for him as he plans to have 'Dr Chilton for dinner.' Lecter is an obnoxious murdering sociopath who we end up liking despite ourselves. So much so, he spawned 2 sequels, a prequel, and a TV series.
Now that is scary.