I think almost any diet works, as long as you stick to it. Ah, there's the rub! It works for a couple of weeks, and then you get bored with it, and then you're right back where you started.
I have been on Atkins for 4 years. It's an atrocious diet (unless you are a dog!), but it fits what I like. It was very hard to give up the potatoes, cereal, corn, pasta, bread, etc., etc., but it allowed my to eat all the meat I wanted (big meat eater), and when I followed it correctly, all my blood work, which was already pretty good, actually improved.
Weight Watchers is probably the best; also South Beach, but they are all about portion control, my arch nemesis. No portion control on Atkins, I can eat half a pound of bacon and not even blink.
The absolute best thing about it is, once I am on it faithfully for awhile, I can gradually add some carbs. I save up all my carbs for daily dark chocolate, and can do so without gaining weight!