...is no diet; it depends on your definition of the word, diet, of course. In this sense, diet means a short term eating plan with short term results to match.
The perfect diet is the one that includes all the food groups in proper amounts to effectively nourish the body with all the essential nutrients. This will be different for each person according to their stage of life, energy requirements and eating styles.
So, spend your time and money wisely, on healthy whole foods. To take it a step further, think about seeing a registered dietitian for your individual needs. The information you gain will be priceless, as this is knowledge you will have for your best health possible.
Yes, you are most certainly allowed to have a treat...just do not make a meal of it. Please remember that this is not deprivation; rather, a part of your healthy lifestyle to suit your taste.
Therefore, the best or perfect diet is a healthy way of eating that can be done for the rest of your life.