Fad Diets
But I know this person who lost 30 pounds on the Adkins diet...  Oh ya?  Just wait...

Why dedicate so much time and energy to a diet that is only temporary?
The HCG Diet - What are you, a squirrel?
500 calories a day is NOT SAFE, and not something you can maintain for a long period of time.  You are going to be starving all day long, your energy levels will be ridiculously low, and you're going to feel like crap.  Doesn't that sound fantastic?!

The Atkins Diet
This diet can be OK when followed correctly, but does not provide an effective long-term solution for weight loss.  Remember what I always say, your weight is only ONE of the values you should be measuring for overall wellness.  You need carbohydrates for energy to work out or perform athletic activities.  Without carbs, your workouts will suffer and it becomes easy to get frustrated.  If you do work out while on the Atkins diet, be prepared for your body to start CRAVING carbs.  It's much better to eat clean and exercise than eliminate something from your diet that your body needs for optimal performance.  The real downer is this:  MOST people who reintroduce carbs to their diet after losing weight with the Atkins diet gain all the weight back.  FAST.  Like, REALLY FAST.  So be prepared to yo-yo if this is your diet of choice.
Links
US News - HCG Diet Dangers: Is Fast Weight Loss Worth the Risk?

Web MD - The Truth About HGH For Weight Loss, 6 Things You Should Never Do To Lose Weight