In Paranormal Activity 4, Katie and/or Toby attempt to asphyxiate Alex by trapping her in the garage and leaving the (locked) car running. After many unsuccessful attempts to open the doors or cry for help, Alex grabs her father's golf clubs, breaks into the car, and instead of simply stopping the engine, she guns it in reverse to smash through the door and into clean air. More than one audience has cheered and applauded at this 15 year-old's intelligent response.
The boyfriend is a walking, talking crowning moment of awesome. He points out how the kinect detects movement, he helps the family document what happens in their home, and furthermore, he's the smartest in the film with the funniest lines too.
The exorcism scene in #2 is the closest thing to an intense climax that the series has gotten.