Coming in a close second, in a tie: The DASH diet, which stands for dietary approaches to stop hypertension and emphasizes reducing salt intake; and the flexitarian diet, which encourages being a vegetarian most of the time -- but is flexible enough to allow a burger once in a while.
All three of these diets reduce or eliminate processed foods, and stress packing your plate with fruits, vegetables, beans, lentils, whole grains, nuts and seeds.
"I think it's important to note that the top three diets -- Mediterranean, DASH and flexitarian -- all offer variety, flexibility and few, if any, rules," said Gretel Schueller, managing editor of health for U.S. News & World Report, who puts out an annual ranking of diets, in an email.
"All the diets that perform well are safe, sensible and backed by sound science. The diet winners also all provide adequate calories with a focus on vegetables, fruits and whole grains; a modest amount of lean protein, dairy; and an occasional treat," Schueller added.
A panel of 27 experts examined 40 diets and ranked them on several categories: How easy is the diet to follow; how likely is a person to lose significant weight, both in the short and long term; how effective the diet is in preventing cardiovascular disease or diabetes; and the diet's nutritional completeness.
"Generally speaking, the top diets are driven by what you can eat -- not what you can't eat. And right now -- during these stressful times of the pandemic -- that's especially helpful for people," Schueller said. "We want food we can enjoy. And we want food that will maintain our health, maybe even boost our immunity. The top-ranked diets offer this."
Mediterranean diet
In addition to its top spot as best overall diet, the Mediterranean style of eating also captured the blue ribbon in the following categories: easiest diets to follow; best diets for healthy eating; best diets for diabetes and best plant-based diets.
Numerous studies have found the Mediterranean diet can reduce the risk for diabetes, high cholesterol, dementia, memory loss, depression and breast cancer. The diet, which is more of an eating style than a restricted diet, has also been linked to stronger bones, a healthier heart and longer life.
The diet features simple, plant-based cooking, with the majority of each meal focused on fruits and vegetables, whole grains, beans and seeds, with a few nuts and a heavy emphasis on extra-virgin olive oil. Fats other than olive oil, such as butter, are consumed rarely, if at all, and sugar and refined foods are reserved for special occasions.
Red meat is used sparingly, usually only to flavor a dish. Eating healthy omega 3 oil-packed fish is encouraged, while eggs, dairy and poultry are eaten in much smaller portions than in the traditional Western diet.
Social interactions during meals and exercise are basic cornerstones of the Mediterranean style of eating. Lifestyle changes that are part of the diet include eating with friends and family, socializing over meals, mindfully eating favorite foods, as well as mindful movement and exercise.
Best diet for heart health
In the best heart-healthy diet category, the Mediterranean diet shared the winner's circle with the Ornish diet, which was created in 1977 by Dr. Dean Ornish, founder of the nonprofit Preventive Medicine Research Institute in California. Today the diet is offered via the consumer health website Sharecare.
Only 10% of calories can come from fat, very little of it saturated, the report said: "Most foods with any cholesterol or refined carbohydrates, oils, excessive caffeine and nearly all animal products besides egg whites and one cup per day of nonfat milk or yogurt are banned, though the plan includes some seeds and nuts."
While experts agree that the diet can be overly restrictive and hard to follow, according to Ornish it's the only scientifically proven program to reverse heart disease without drugs or surgery, as shown in a randomized clinical trial.
The diet is combined with stress-management techniques, exercise, social support and smoking cessation, which "formed the basis of Ornish's landmark heart disease-reversal trial in the 1990s," the report said.
Best diet for diabetes
In the best diet for diabetes category, the Mediterranean diet took top honors. Tied for second place were the flexitarian and vegan diets.
The flexitarian diet is a combination of two words: flexible and vegetarian. With this diet, you don't have to swear off meat completely. You can be a plant eater most of the time and have the occasional steak or burger. A mostly plant-based diet is linked to overall better health and longevity, the report said, including a lower rate of heart disease, diabetes and cancer.
The vegan diet takes vegetarianism a step further by eliminating all animal products -- even dairy and eggs. That means "no refried beans with lard, margarine made with whey and anything with gelatin, which comes from animal bones and hooves," the report said.
Best overall diet for weight loss
For this category, the popular WW (formely called Weight Watchers) plan tied with the flexitarian and Volumentrics diets for the No. 1 spot.
The flexitarian diet wants you to add foods to your diet such as non-meat proteins like beans, peas or eggs, as well as fruits and veggies, whole grains, dairy and seasonings. Then you try to follow the following calorie counts, the report said: "Breakfast choices are around 300 calories, lunches 400 and dinners 500. Snacks are about 150 calories each; add two, and your daily total clocks in at 1,500 calories."
"If you emphasize the plant-based component of this diet -- eating lots of fruits, veggies and whole grains -- you'll likely feel full on fewer calories than you're accustomed to," according to the analysis of the flexitarian diet. "Research shows vegetarians tend to eat fewer calories, weigh less and have a lower body mass index (a measure of body fat) than their meat-eating peers."
The WW plan, which along with the Mayo Clinic diet tied for first in best commercial diets, has an important component needed in any successful diet, the report said: social support. In addition to in-person meetings and optional one-on-one consultants, the plan offers an online community.
"A backbone of the program is support via the WW app, expert-led workshops and Digital 360 plans to provide practical tools and behavior-change techniques for help along the way," the analysis said.
The Volumentrics diet is all about cutting the energy density (calories) of the foods you eat. One of the best ways is to eat a lot of foods that are high in water, as that increases the weight of food in your stomach without packing in additional calories. Top choices include soups, which the report said is 80% to 95% water; fruits and vegetables which are 80% to 95% water and yogurt, which is about 75% water.
Best diet for quick weight loss
The quick weight loss category is for the person who needs to lose a few pounds for a special occasion. These eating plans are not recommended as a long-term manner of eating, according to U.S. News & World Report.
The Atkins Diet, created in 1972 by cardiologist Dr. Robert Atkins, took first place in this category. The diet involves four phases of meal plans, starting with very few carbs and adding more as the diet progresses.
However, "low-carb dieters may eat too much fat, raising health concerns," the analysis found, giving the diet a 34 out of 40 in the overall rankings.
The Health Management Resources Program, or HMR, tied for second place with the OPTAVIA diet.
The HMR program is a weight loss and lifestyle-change program designed to reduce calories via meal replacements that include added fruits and vegetables. The two-week starter kit is just shy of $200 and includes 70 servings of HMR shakes and entrées, support materials, weekly group coaching and free shipping, the report said.
The OPTAVIA diet wants you to buy its "Fuelings" and supplement them with a "Lean & Green" meal (one meat, veggie and healthy fat entrée) each day.
"Between 10 to 35 percent of daily calories come from protein," the report said, which is more than government nutritional guidelines suggest. "Prices for OPTAVIA's Optimal Weight 5&1 Plan range from about $400 to $450 for 119 servings, or about three weeks' worth; that's about $13-$15 per day."
Both HMR and OPTAVIA came in at 27 out of 40 diets in the overall ratings, according to the report.
Coming in last
The Dukan Diet landed at the bottom of the rankings of best diets. Other poor performers included Whole30, which tied for 35th place and the popular keto diet, which tied with the modified keto diet for the 37th spot.
The Dukin, Whole30 and keto diets, which emphasize eating high-protein or high-fat foods with minimal carbohydrates, are typically rated poorly by experts because they are extremely restrictive, hard to follow and eliminate entire food groups, which is not recommended by dietary guidelines.
Getting started on the Mediterranean diet
Want to make the Mediterranean diet one of your goals this year? Get started by cooking one meal each week based on beans, whole grains and vegetables, using herbs and spices to add punch. When one night a week is a breeze, add two, and build your nonmeat meals from there.
Grains that have changed little over the centuries, known as "ancient grains," are also a key feature of the Mediterranean diet. Quinoa, amaranth, millet, farro, spelt, Kamut (a wheat grain said to be discovered in an Egyptian tomb) and teff (an Ethiopian grain about the size of a poppy seed) are some examples of ancient grains.
When you do eat meat, have small amounts. For a main course, that means no more than 3 ounces of chicken or lean meat. Better yet: Use small pieces of chicken or slices of lean meat to flavor a veggie-based meal, such as a stir fry.
And rethink dessert. Mediterranean cultures typically close out meals with fruit that's in season.
If you tire of eating raw fresh fruit, get creative. Poach pears in pomegranate juice with a bit of honey, then reduce the sauce and serve over Greek yogurt. Grill pineapple or other fruits and drizzle with honey. Make a sorbet out of fruit, including avocado (it's really a fruit). Stuff a fig or date with goat cheese and sprinkle on a few nuts. Create a brown rice apple crisp or even a whole-wheat fruit tart.