7. Martha Marcy May Marlene
The Ending: Martha, who has spent the movie on the run from a cult, returns to her family and experiences paranoia-fueled visions of her captors, apparently trying to re-capture her. After noticing a vision of a man on the lake - who may or may not be real - the same man runs in front of the car being driven by Martha's brother in law. The man then gets into a parked car behind their own car, and appears to follow them, before the movie cuts to black.
Why It's Infuriating: It's a madenningly ambiguous note on which to end the film; it's fashionable these days to be open-ended and leave things up to the viewer, but after 2 hours of subjectivity, a little closure would have been nice. Instead, Martha's delusions simply continue (perhaps), and we're left to debate until the end of time whether Martha and her family were followed and murdered - or perhaps Martha was returned to the cult - or if this is just another of Martha's visions.