My only vote goes to Blair Witch - nothing happens apart from a few kids get lost for a few days in the woods and hear some nature sounds and they make up the rest.
Nothing out of the ordinary happens, nothing unbelievable, that is. No suspension of disbelief needed. No zombies, slashing baddies, recurrent baddies who can't be killed, no magic or voodoo. Basically, all of the fear is up to you. Is getting lost in a very big woods for days and going in circles inherently, intuitively scary? I say it is and when I see people doing just that, I can empathise with them and I get scared too.
Apparently, people think something silly happens in the last few frames, but I've never seen it. Maybe it is there, but if it is, I think the film is all the better for not seeing anything.