Mrs. Faithful, I'm not sure if you are asking what is a good "diet plan" -- meaning a specific diet like Atkins, South Beach or (god forbid) The Master Cleanse; or whether you are generically referring to a "good diet" in the sense of what issensible, healthy and sustainable over the long-haul when it comes to nutrition and healthy eating habits.
If you are referring to the first scenario, as some of the answerers have already pointed out, temporary, quick-fix crash or fad diets are basically a one-way road to fat-loss and weight-loss frustration. No one should "go on a diet" -- they should instead learn about nutrition and healthy eating and make a lifestyle change (and often that's what it takes) to eat smart for their entire life.
Consider this: According to the US CDC, less than 10% of people who are "dieting" will experience meaningful, long-term weight loss. Those are pretty poor odds, and they say something about the types of eating plans people are choosing.
In my experience, the best "diet" is one that focuses on whole foods that are as close to how they grow in nature as possible. Avoid pre-prepared, heavily processed foods, fast food, excessive sugars and saturated fat.
Learn to cook for yourself, which is one of the most important skills if you want to eat healthy for the rest of your life. This approach is also known as "Clean Eating", which is one of the most effective, and sustainable approaches to improving your health and losing fat.
There are very few people who have adopted this approach who don't have dramatic results, although those results appear over months, rather than days (unlike fad or crash diets.) And because it's a change in your eating habits, Clean Eating isn't prone to the "rebound effect" of crash diets.
Try it and let us know how it works.
Best of luck -- Matt
