ONE food? Are we talking a natural single food source or something processed with many food sources in it?
If we're talking natural, I think some key things to look for are vitamins, protein, and trace minerals.
I'd personally choose something like Quinoa or Winter Squash (pumpkin, hubbard, etc) - Although winter squash has very very little protein, it makes up with several minerals, nutrients, and the much needed vitamins as well as energy/carbohydrates and even calcium. Quinoa on the other hand is very high in protein and minerals, but has no Vitamin C, but still has other essential vitamins.
Both store well.
Either one though, a diet of strictly that, I think they'd have unhappy lives.
If we're talking a processed food - I'd look for something with plenty energy/fat, protein, vitamins, and minerals. You know, something with everything.
However many vitamins have a terrible shelf life and a sensitive temp. tolerance.
If you're prepping for your chickens, you'd want to basically buy extra feed, a flock raiser or all-in-one. Storage would be an issue, it would need to be dry and rodent-proof. If you're SERIOUSLY prepping for your chickens, remember chickens lived most of the past without "feed", they just free-ranged or lived off scraps. They're omnivores, just like humans, so can eat pretty much what we do. So, what would you be able to grow in your area to sustain yourself and your chickens?
If the world was coming to an end, I would not buy food for my chickens because they would not need food when the world is done .
Ditto Here, Welcome from Ohio, u Joined a great Forum here! What if you only had beans to eat the rest of your life,, no I take that back I could get used to beans and corn bread every day, Maybe some buttermilk on the side " For the CornBread" KornBread ummm!