The answer - it depends - everyones body is so different and there isnt really one 'diet' that will help people lose weight or become fit.
However, the general consensus now is that sugar is the cause of weight lose (rather than fat in the past), and so avoiding as much added sugar is going to help, as sugar is just 'empty calories'. New research suggests that fats arent as bad as they once expected (as long as they are from natural sources) - and people even report weight loss and other benefits from high fat/low carb diets suchn as ketogenic diets.
The debate about carbs seems ongoing, but to many 'starchy' carbs such as potatoes, bread and pasta seem to make people bloated, and can lead to other problems such as IBS.
The main thing to do is have a 'balanced' diet, rich in fresh vegetables, fruit and healthy sources of protein (doesnt have to be meat if you are vegetarian). A good mix of healthy fats, protein and carbs is probably best. Fats will keep you satiated and not hungry, protein will help with muscle growth, and carbs provide us with energy.
A good overall diet is the paleo diet, although it can be a bit restrictive at times I think it is a good guide as to what will keep us healthy overall