Among the countries you listed—United States, Chad, Sierra Leone, and Mali—it's most likely that the United States, under current dietary trends, would be in the bottom three countries for healthiest diets. While the U.S. has access to a variety of healthy food options, diet-related health issues such as obesity, diabetes, and heart disease are prevalent.
Chad, Sierra Leone, and Mali, while facing their own challenges, often adhere to more traditional diets that may be lower in processed foods and sugar, which could contribute to healthier eating patterns in some respects.
So, the likely bottom three in terms of healthiest diets would include the United States and possibly Chad and Sierra Leone or Mali, considering various health and dietary factors.