Scariest / most upsetting movie you've ever seen?

Last-house-on-the-left_medium

So, I'm interested to know what you guys think is the SCARIEST or most UPSETTING movie ever? I'm not talking about the most gory or nasty, but the one that left you most freaked out. I'm voting for David Lynch's later work (Lost Highway, Mulholland Drive), or something like Last House on the Left (Laura reminded me of this one, the original). I also have to say if you really want to have the worst experience ever, watch Irreversible (just kidding don't watch it, it ruins your life).