What's more important?
Seeing the number on the scale go down--which you can achieve by going on a "diet", even if it's a diet of junk food? Or seeing your BMI number go down--which you can easily achieve by going on a starvation diet, except that you'll probably lose a great deal of muscle instead of fat?
Or totally reconsidering the quality (rather than the quantity) of the food you eat? (Warning: you might end up eating less food overall if the quality of your eating improves.)
Or reading the nutrition labels on food you buy in order to find out about nutritional content and preservative content rather than simply the number of calories?
Or shopping more in the fresh produce aisle?
Or finally understanding why you feel you have to eat a loaf of bread at one sitting or a bowl of cookie batter? (Warning: you may have to leave a bad relationship if you do.)
Or choosing to go for a walk every day?
Or choosing to take public transit rather than the car?
Or going to your local community centre and going for a swim (cost: a few dollars--a lot less than a gym membership)?
What's better? A healthier lifestyle that doesn't focus on weight? Or getting those numbers down, come hell or high water, no matter what the actual cost to your health?