U.S. News and World Report this morning issued its first-ever assessment of 20 popular diet programs and dietary “approaches,” naming the DASH diet the best overall and Weight Watchers the best of the commercial diet programs.

The online news magazine assembled information about how each diet works, whether its claims stood up under scrutiny, any potential health risks it poses and what it’s like to actually live on the diet. A team of 22 experts then graded the diets for their short- and long-term weight-loss results, how easy they were to follow, their nutritional quality (as measured by compliance with the 2010 Dietary Guidelines for Americans), their safety, and their ability to prevent or manage diabetes and heart disease.

Here’s how the news release announcing the report summarized its findings:

— Best Weight-Loss Diets

Weight Watchers ranked No. 1 in weight loss. Tied at No. 2 were Jenny Craig and the Raw Food Diet, an approach that challenges dieters to avoid foods that have been cooked.

— Best Heart-Healthy Diets

The Ornish Diet ranked No. 1 for heart health. The TLC Diet, a government-designed eating plan that stands for Therapeutic Lifestyle Changes, ranked No. 2. Another government-developed diet, DASH (Dietary Approaches to Stop Hypertension), ranked No. 3.

— Best Diabetes Diets

The DASH diet ranked No. 1 for preventing or managing diabetes. Close behind, in a three-way tie at No. 2, were the Mayo Clinic Diet, the Ornish Diet and the Vegan Diet.

— Best Diets Overall

The DASH diet ranked No. 1 overall. Three diets tied at No. 2, excelling in all measures U.S News considered: the Mediterranean Diet, the TLC Diet and Weight Watchers.

— Best Commercial Diet Plans

Some dieters may seek the structure and social support provided by many brand-name programs, so U.S. News also examined how eight prominent diets stacked up. Weight Watchers ranked No. 1, Jenny Craig ranked No. 2, and Slim-Fast ranked No. 3.

The report counters the rankings issued last month by Consumer Reports in which Jenny Craig came in first, with Slim-Fast in second place and Weight Watchers third. CR’s method attracted some criticism, as reported in this blog.

Are you on a diet right now? Which one, and how did you choose it?