TIE - 8. Watercress

Watercress, with its little round leaves, was considered the top powerhouse food in the CDC study. However, by our metrics, it didn't pack in as many nutrients as others on the list. It's high in vitamins A, C, and K and incredibly low in calories.

It's also linked to a lower risk of type-2 diabetes and is not too hard to grow.

Calories per cup: 4