The Mediterranean diet is the healthiest in the world for the 7th year in a row. Here's why.

• The Mediterranean diet has been ranked the healthiest way to eat by US News & World Report.
• The eating plan is flexible, full of whole foods and plants, and is linked to longevity benefits. 
• Experts say the best diets are easy to follow, not restrictive, and focus on unprocessed foods. 

The Mediterranean diet continues its undefeated streak as the ultimate choice for healthy eating, winning the top spot overall for the seventh year in a row, according to annual rankings from US News & World Report.

It emphasizes whole, unprocessed foods, healthy fats, and plenty of plants, with ingredients like olive oil, whole grains, beans, seafood, leafy greens, and other veggies.

The diet rankings for US News & World Report are conducted annually by a panel of experts including doctors, dietitians, and weight-loss researchers.

They evaluate popular diets, including commercial programs like WeightWatchers, based on factors such as nutritional completeness, long-term sustainability, and how easy it is to follow. The resulting analysis is used to create rankings for 11 different categories, ranging from "Best for Weight Loss" to "Best for Diabetes" to "Best Family-Friendly."

This year, the Mediterranean diet dominated, ranked best overall by a significant margin, and took the top spot in seven other categories.

Why is the Mediterranean diet so healthy?

The Mediterranean diet is based on traditional eating habits in parts of Greece, Italy, and Spain, and is rich in foods like olive oil, whole grains, nuts and seeds, legumes, and fatty fish.

Experts gave it top marks for being heart-healthy, easy to follow, and rich in nutritious food options.

Part of the diet ranking process is evaluating the evidence of a diet's potential health benefits (or side effects), such as scientific studies.

Extensive research on the Mediterranean diet suggests it may improve heart health, support weight loss, and even prevent cognitive decline.

The diet also overlaps with eating habits for longevity. Several Mediterranean regions are known as Blue Zones, areas of the world where people live the longest, healthiest lives, according to researcher Dan Buettner.

So-called SuperAgers who live to 100 or older often follow similar principles as the Mediterranean diet, eating lots of complex carbs like beans and healthy fats like olive oil.

What diet is better than the Mediterranean diet?

Beating out the Mediterranean diet for the top-ranking spot is a tall order.

The DASH diet is one contender: ranked second place overall this year, some experts have suggested it might be even healthier because it offers more guidance on salt and alcohol consumption, according to the American Heart Association.

For the US News & World Report experts, the Mediterranean diet still won out in categories such as being easy to follow, family-friendly, and focused on plant foods.

But there is potential for another diet to take first place, if it were to combine the best parts of the Meditteranean diet while making up for its major downside, Gretel Schueller, managing health editor at U.S. News & World Report, told Business Insider last year.

She said many of the healthiest eating plans are similar to the Mediterranean diet in that they prioritize whole, unprocessed foods, avoid restriction or rigorous calorie counting, and have a balance of healthy carbs, protein, and fats.

However, there is one area where the Mediterranean diet falls short, at least as it's typically followed — all that flexibility can leave you to make a lot of decisions on your own, which can be trickier than some of the focused coaching and group accountability of other programs.

"All the top diets check those boxes," Schueller said. "Maybe we'll get the best elements of community support and the Mediterranean diet."

What is the worst diet?

The Mediterranean diet gets points for its flexible, unrestrictive approach to eating. In contrast, some of the lowest-ranked diets are strict, making them difficult to sustain over time, according to the panelists.

Trying to ban specific foods or cut out entire food groups can backfire by setting you up to crave those foods even more and eventually drop the diet, in addition to missing out on some nutrients.

But the rankings also reflect that stricter eating rules may lead to results at least in the short-term: the keto diet was awarded first place for quick weight loss, followed by the similarly low-carb Atkins diet. Both plans drastically cut carbohydrates in favor of dietary fats.

Previously, the keto diet has also ranked the worst overall, based on concerns that it can cut out potentially healthy carb sources like whole grains and fruit, and may include too much saturated fat. Recent research has also suggested keto may be a viable option for diabetes treatment, though, and it may be possible to follow a healthier version of keto that includes leafy greens and other low-carb produce. As such, experts have given keto slightly better marks than before.

Now, the dubious honor for the lowest ranking goes to the raw food diet — which is exactly what it sounds like. It received low marks because it is unnecessarily restrictive and difficult to follow long-term.

Others in the bottom five include Herbalife and Slimfast programs which rely on processed shakes and bars instead of healthy habits, and the Dukan diet, a trend with rumored celebrity fans and periods of intense restriction.