It is particularly important to maintain a well-balanced, low-fat diet that is both rich in whole-foods and fiber AND low in processed food and saturated fats.  The challenge for those with PCOS is that weight loss is more difficult because elevated androgens increase appetite with intense food cravings and insulin, which is a growth hormone, promotes weight gain, especially in the abdomen.
While there is no single best diet for PCOS, some diets such as the Mediterranean diet are rich in vegetables, fruits, nuts, and whole grains, which are well balanced. Intermittent fasting may be considered if you are not trying to get pregnant.  Even a weight loss of only 5-10% may reduce androgens and help restore normal ovulation.