The best:
Two-way tie between "Alas Babylon" and "Lucifer's Hammer" with "Babylon" being the more realistic and useful at a practical level but "Hammer" being the more exciting.

Not too bad but lacking: "The Keep", which get tiresome because of the protagonists political and economic grumblings, and "Farnham's Freehold" which veers too quickly into sci-fi fantasy.

Low middling but with some merit: "Patriots". Weakly inspired writing made worse by patently false and dangerous assumptions. The book isn't entirely useless but you need to take the assumptions, setting, action and techniques with a large grain of salt. Borderline gun porn with an overweight emphasis on firearms and violence.

The worse: "The Survivalist" series by Jerry Ahern. This series is little more than comic-book heroic action, gun porn and posed self-assumed superiority. The protagonist never pulls a generic gun. He pulls a particular weapon. Often described with pornographic detail including make, model, manufacturer, barrel length, trigger pull, and everything short of the serial number. Violence and degradation of other, lesser survivors, is described in gloating detail. All the better to highlight the assumed nobility of the protagonists I suppose.

Within the course of the individual books and the series, as a whole, the violence and self-righteous posing escalates to wild excess while the plot line and prose largely repeats itself with just a few embellishments and escalation. Pretty much like other forms of pornography.

Unknown to me but highly reviewed: "The Road". It is "on-deck" as my next purchase.