Scariest movie ever: Phase IV?

It occurred to me that after The Andromeda Strain and The Satan Bug had already cropped up on the blog as examples of truly scary movies, it might be interesting to work out what the scariest might be. Anyone got any further suggestions? I’m sure The Exorcist will be in there somewhere, but Phase IV – the one with the ants – has got to be in there (I was minded of it by Marie’s current problems). 

Directed by Saul Bass and featuring some stunning micro-photography of real insects, it asks the question: “What if ants suddenly became intelligent, stopped fighting each other and decided to gang up on us for their own sinister purposes?” Non-stop scary and very weird.