There's as many diets as there is dieters and, all of them are the best in the world, until they ultimately fail...
Loosing weight is easy; keeping it off is the hard part. How many peoples play "yo-yo" with their weight, going from one diet to the other? There's a shitload of these and MD will mostly agree yo-yo is worst than any stable weight, too high or not. Some peoples since 20 years were on a diet when I met them and they're still on a diet, with the weight still going up and down but an average that's still too high. If any of these diets would have worked, they'd be skeleton-thin now!
Some diets are downright dangerous. Montignac and Atkin's being two of these. Why? They aren't balanced and result in too much intake of some stuff while depriving of other. No food is absolutely bad or good, it's the quantity and proportions that make the difference. You could die because you only eat bacon but you could also die because you eat only carrots!
A diet need to be balanced and, to have any form of success, it must aim towards a way of eating daily, not a way to loose weight fast. It must also be easy to follow and not too limiting.
Any diet without a mindset on the long term will fail. Any diet that doesn't implicate a strong will to loose weight and keep it off by the dieter will fail and, the long term is the enemy since good will is something that tend to fade in time...
Let's face it: we eat too much, with sugar and fat in way too high proportion compared to healthy food and, last but not least, we don't move enough.
I know of a few peoples who follow Weight Watcher since a few years and all of them lost about 15 pounds within about 1.5 year and are maintaining this new weight relatively stable. It's not easy! As soon as they "forget" to count points, pounds come back (1 or 2) but, when they start paying attention, those pounds go away again. None of these peoples do more exercise than they used to do but what they eat, what proportions and how much is adapted to their lifestyle. They can eat whatever they want and have a "points reserve" per week for special occasions. They even turned the "point-counting" into a game, competing together on who could follow as close to the calculated allowance, without getting over or under.