Adherence to diet for one year, not the particular diet plan, would be the most important determinant of effective weight reduction. According to the responses of a randomized trial published in the January 2005 issue of JAMA. When looking at Atkins, Ornish, Weight Watchers, and Zone diets, the author recommends the "low fad" approach.

The accessible Fad Approach

The accessible Fad Approach
Effective fat loss is best achieved by using the accessible fad approach. The low fad approach to losing lose weight fast no diet involves reasonable reductions in caloric consumption with increased physical activity. There's nothing "fad" regarding this approach and is supported by just a few studies.
The fact that the accessible trend strategy is supported by scientific evidence is key...the fad diets already in the market have little or no credible proof supporting the claims of theirs.
"The scarcity of facts addressing the health effects of diets which are popular is a crucial public health concern, especially since patients as well as physicians are interested in using trendy diet programs as individualized eating strategies for disease prevention," write Michael L. Dansinger, MD, a Preventative Medicine professional from Boston, Massachusetts.
Dr. Dansinger passes to tell you, "Some plans reduce carbohydrate consumption with no extra fat restriction (eg, Atkins diet), many modulate macronutrient balance and glycemic load (eg, Zone diet), and others restrict body fat (eg, Ornish diet)."
And so, in addition to the shortage of proof backing the claims made by fad diets, there is no consistency from one diet to the next within the same group of diet type. This makes it very hard to study the fad diets and measure the effects on your health, let alone your chances for successful weight loss.
But there's hope for dieters...the low fad approach or "Low Fad Diets."
Successful Weight Loss...The Evidence Supporting Low Fad Diets

Profitable Weight Loss Conclusion