This is an excellent writing question. I lean toward an EMP with some crazy mixed-up events that further complicate everything for everyone – like some sort of accidental weapon exchanges. Settings can be very interesting any way you go but suffice it to say that most people don’t have much survival skill anymore anything will present many challenges. When is a major factor. Generations past the event steals from the tension of event which can heavily lend itself to the setting, almost becoming a character unto itself.
Have a look at C. S. Boyack’s thoughts on the subject.