Vegetables can be a great asset in weight loss because they provide a larger portion with a fewer number of calories. However, just like with anything in life, all vegetables aren't good for you. Vegetables that come in cans can be especially troublesome because they tend to have higher sodium content.
You are generally ok buying fresh or frozen vegetables because they tend not to be covered in dressing or sauces. Vegetables can also lose their "good" qualities depending on the cooking method you use, methods like frying is always discouraged.
The vegetables that my friends and I have the most success with is spinach and cabbage. They are extremely low-calorie and can be prepared in variety of ways from raw, steamed, or cooked in soup. According to the American Heart Association a diet high in produce will significantly lower a persons' risk of cancer, heart disease, and diabetes.