According to Science Daily, reporting on a UCLA researchers report in the April issue of American Psychologist, "....the majority of people regained all the weight, plus more. Sustained weight loss was found only in a small minority of participants, while complete weight regain was found in the majority. Diets do not lead to sustained weight loss or health benefits for the majority of people." See the 'Dieting Does Not Work' link to the right under Credible Evidence.
Of course everyone is on a diet if you define 'diet' as "the sum of the food consumed" as opposed to "the deliberate selection of food to control body weight".
In my estimation gleened from reading and perusing various books and studies, the BEST diet is one rich in the needed nutrients yet which limits the caloric intake to the amount your body burns off. A very common imbalance occurs when, as we in the western culture tend to do, eat way too much highly processed (read 'mostly devoid of nutrition') junk foods and drinks thereby gaining more calories than the nutritional content justifies. And whether the food intake is healthy or unhealthy (as expressed in the ratio of nutrition:calories), we also tend to eat too much making it difficult for our bodies to burn off the calories given our chosen exercise level. So I say, the best diet is composed of healthy foods in smaller quantities than we're use to accompanied by adequate exercise.
And 'diet' should not principally be a weight gain/loss device. It is the right thing to do for our wellness. The out of control diabetes, heart disease, and cancer rampant in our society has occured largely since falsly devised and highly promoted 'low-fat low cholesterol" diets have been pushed on us. Folks, they're not our healer, rather our downfall!
For a good look at the highly complex and interactive thing we call our body and it's needs and reactions to what we eat, I recommend reading books by Dr. Diana Schwarzbein MD. She has written several on "The Schwarzbein Principle"
(see http://www.schwarzbeinprinciple.com/ ) Good Stuff for all!
And if you're into 'dieting', please read the article in Science Daily.