If it is a single item food, I'd have to say yams or cattail.
@roamer_1 I am not surprised you picked yams as you are a smart person. In this country, yams is another name for sweet potatoes. One could live on sweet potatoes and nothing else and still be healthy. They are essential to store for emergency food.
"Health benefits of Sweet potato
Sweet potato is one of the high-calorie starch foods (provide 90 calories/100 g vis a vis to 70 calories/100 g in potato). The tuber, however, contains no saturated fats or cholesterol, and is a rich source of dietary fiber, antioxidants, vitamins, and minerals than regular potatoes.
Its calorie content mainly comes from starch, a complex carbohydrate. Sweet potato has higher amylose to amylopectin ratio than that in regular potatoes. Amylose raises the blood sugar levels rather slowly on comparison to simple fruit sugars (fructose, glucose, etc.) and therefore, recommended as a healthy food even in diabetes.
The tuber is an excellent source of flavonoid phenolic compounds such as beta-carotene and vitamin-A. 100 g of tuber provides 14,187 IU of vitamin-A and 8,509 µg of ß-carotene, a value which is the highest for any root vegetables. These compounds are powerful natural antioxidants. Vitamin-A is also required for the human body to maintain the integrity of mucosa and skin. It is a vital nutrient for healthy vision. Consumption of natural vegetables and fruits rich in flavonoids helps protect from lung and oral cavity cancers.
The total antioxidant strength of raw sweet potato measured in terms of oxygen radical absorbance capacity (ORAC) is 902 µmol TE/100 g.
The tubers are packed with many essential vitamins such as pantothenic acid (vitamin B-5), pyridoxine (vitamin B-6), and thiamin (vitamin B-1), niacin, and riboflavin. These vitamins are essential in the sense that the human body requires them from external sources to replenish. These vitamins function as cofactors for various enzymes during metabolism.
Sweet potato provides a good amount of vital minerals such as iron, calcium, magnesium, manganese, and potassium that are essential for enzyme, protein, and carbohydrate metabolism.
Sweet potato top greens are indeed more nutritious than the tuber itself. Weight per weight, 100 g of fresh leaves carry more iron, vitamin-C, folates, vitamin-K, and potassium but less sodium than its tuber."
@CatherineofAragon