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.