I think most folks are going to put Stephen King's The Stand at or near the top of the heap. Your mileage may vary.
My personal favorite would be Harry Harrison's Make Room! Make Room!, the novel that was the basis of the film "Soylent Green."
I'm also responding to put in my personal recommendation that if anybody suggests Earth Abides, you run screaming in the opposite direction. I found it unacceptably racist, even when I attempted to view it in the light of the times in which it was written.
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