In the ultimate nightmare scenario, there is no safety. Gold was confiscated by FDR. The value of cash can be wiped out in weeks if hyperinflation kicks in. Bonds default. Stocks crash. Businesses go under. Housing prices (in general) dropped sharply during the Depression. Commercial real estate investors seem to pop in and out of bankruptcy every other year.
But, bottom line, if I picked something to own in the nightmare scenario, my first choice would be a fertile farm in a hidden valley. My second choice would be a mortgage free home.
We are in a dangerous time. I'm not sure if we could survive something like the Great Depression today. In the 1930's, most of the country was on the same page from a moral, religious, and cultural viewpoint. And we almost didn't survive then. I hope to never see a simultaneous collapse of stocks, bonds, real estate, and (I would guess) everything else. It wouldn't be pretty.