There is no single thing.  When scientists started looking around the
world at food traditions they spotted food patterns, which in
combination provided a sound nutrition.  For example, the native
american diet in many places where farming was done was based on
beans, corn, varieties of squash and other fewer veggies and some
meat. In some places such as the astecs in mexico meat was rare as a
common food and used the corn/bean/squash combination almost
exclusively. The rice, soy bean, veggie combination with little meat
is an example in the e. asian cultural area.  These provided less
complete sources of proteins that when combined were complete and the
veggies and other such provided vital vitamin/minerals when combined.
Potatoes were part of such a combination in s. america and in the
irish case.  The potatoe is a fine source when all of it is used,
excessive carbs or any other macro source only becomes a problem if
too much of any one is used to the detriment of some vita/minerals and
other nutritional factors such as antioxs etc.; and of course the real
problem is eating too much such that calories eaten exceed the amount
of energy required for a given level of physical activity.

Not a food, but hundreds of foods, since diversity is protective
and our proto-human ancestors and early man likely had access
to several hundred different types of wild plant foods, along with
modest quantities of animal foods, mostly invertebrates in the
earlier days. Mostly or all raw. Cooking tends to destroy the
protective phytochemicals of plant foods, and recent studies have
found that while raw vegetables are most protective for certain
types of cancers, cooked vegetables can yield a null effect.

But, if you seek the single cultivated plant food that has the
highest nutrient density (considering only vitamin and mineral
DRIS) the winner is Swiss chard. Not that you should eat it in
abundance, though, because it is also high in oxalic acid, which
will inhibit absorption of some of the minerals.

Regular potatoes are not as protective as non-starchy vegetables
or even other starchy vegetables such as the sweet potato or
winter squash. Some nutritionists think that regular potatoes
should be excluded from the recommended 5-7 servings of
fruits/vegetables per day.

You could do far worse. You could also do better.

It's an oversimplification. You shouldn't limit non-starchy
vegetables, eat as much as you can, different ones, preferrably
raw. For starchy vegetables, which are not as protective as
non-starchy ones but provide a more concentrated source of
calories, try to choose the more colorful ones. Some can be
eaten raw if shredded or grated. All diets, regardless of
macronutrient ratios, appear to be healthiest when the soluble
fiber fraction is high to very high, but this usually also means
a moderate to high total carbohydrate fraction. Depending on
how one defines high.

Some chinese believe you can exist solely on a diet of rice and chinese
broccolli.

Hi Gabe.
For survival I would suggest that meats and fish would give you
everything your body needs to survive. And to reuse the minerals I
would drink my own urine.

But to be health long term, meat is too powerfull for our health. It is
like a fire you need for survival. The best fastest and easiest is
kindling, but on a long term, it is better to use branches.
I wrote a paper listing some of the sources of foods from the USDA 15
foods data base which are the richest sources for each each of the
essential  nutrients that our body has to get from food. Most of the
food sources are plants that grow world wide and which have been used
for centuries.

Meats and fish on top of this food pyramid are the most complete
sources as they are the most concentrated. Because of this they contain
too much of the types of fats that are harmfull in too large amounts
because they make us too fat to remain healthy.

Eggs and milk, are one level lower and being produced by meat bearing
animals are very complete and more healthy. Butter and cheese are
concentrates of milk.

Seeds are in the middle of the pyramid and feed the animals that
produce the meats. They produce less of the harmfull fats and more of
the necessary essential ones.

Vegetables and fruit are one level lower and one level healthier.

Herbs and spices (leaves) that produce the vegetables and fruits are on
the bottom of this pyrimid. Different plants offer different nutrients
so it is important to have a wide variety for completeness.

For long term survival and health I would base my nutrition on mainly
plants and their fruits and seeds. I would try to find birds and their
eggs to eat. It would be nice to have a goat, not to eat, but to keep
for milk and cheese. I would learn and occasionally go fishing and
hunting. And I would drink my own urine.
use as wide variety of foods available as possible and would try to
The more varied
You can read this paper at
http://www.geocities.com/andrewvecsey/optimalhealth.html
regards
andrew vecsey

Your question aroused my interest and i wrote a paper dealing with it.

http://geocities.com/andrewvecsey/optimalhealth.html
Abstract:

Optimal health is defined. Suggestions are presented as to how it can
be reached despite the many obstacles to it that must be overcome. The
presently known essential nutrients are listed with foods that are rich
in these nutrients. Correcting diet deficiencies using these foods is
highlighted. Analogy comparing the biological functioning of our body
to the functioning of machines and communities are presented. They are
used for building an understanding and appreciation of the important
factors that hinder or help us in attaining and maintaining optimal
health. Part 1 deals with general health concerns. Part 2 introduces
molasses, yeast, cod liver oil, flax, and wheat bran as foods that can
replenish the depleted nutrients in foods caused by processing. Options
using commercial meal replacement products and multi vitamin /mineral
capsules are compared. Part 3 deals with symptoms of deficiencies and
choosing the right foods to cover these deficiencies.
regards
andrew vecsey