I think the most probable SHTF scenario where survival is a concern involves the exponential decline in the stability of our country or region due to competition for diminishing resources both worldwide and on a local scale. I think in my lifetime (in my 20s) we will see constant world wars fought over resources. I think that horrific struggles over energy production/distribution, water, and food will all be contested. I am not sure which will happen first but they are all connected and one affects the others. It's also very possible that differing circumstances will yield different priorities as to which resources are worth fighting over.