10 (tie): Vegetarian diet
The vegetarian diet is simple: no meat allowed. Ideally, meat is replaced with other sources of protein, as well as fiber-packed veggies, fruits, and whole grains to help keep you feeling full.
Unlike a vegan diet, which ranked 19th on the US News & World Report list, vegetarians can eat animal products like milk and eggs, which can be good sources of protein.
[Photo via @cleaneatsbytay]