None of them and eating too little teaches your body to manage on less calories, so making it easier to gain weight in the future.
Unless you change your eating habits for life you will always be battling with your weight. Be patient, don't get discouraged. There is no quick fix that lasts.
For Nutrisystem, it will cost you about $54 and you can save a little bit more if you sign up for an auto ship. It is possible to spend over $400 a month. HCG does nothing, and beware Herbalife..."They don't care who they sell their products to and when recruiting want nothing more than for you to sign up and spend money on buying thousands of dollars worth of stock."
If you eat 3,500 calories more than your body needs, you will put on about 1 pound. If you use up 3,500 calories more than you eat, you will lose about 1 pound in weight.
Daily you need...
http://caloriecount.about.com/cc/calories-burned.php
To burn calories...
http://www.prohealth.com/weightloss/tools/exercise/calculator1_2.cfm
If you don't mind a little extra work, then you are quite capable of doing your diet all on your own...
http://www.caloriecounting.co.uk/resources/intro.htm
...note, you burn calories shopping for food, preparinh it.