Fortunately, you've come to the right place; we're going to answer that in the next 400 words.
What everyone agrees on is that the dieting sector is huge and that there is no definitive list of all the diets on offer. Even if we could link to a list, it would be out of date a week later; if not a day as so many ‘fad’ diets come and go.
'Not' the best diets!
In answering what is the best diet, we can say what it isn't.
It isn't bingeing on one low calorie food at the expense of all others (cabbage soup, anyone?) It isn't cutting food intake to such an extent that you lose weight but feel listless and hungry, forcing you to quit the diet, then see a return of all the weight, and sometimes more.
For something to be the best diet, or even just a good diet, it has to do more than simply get initial weight off. The evidence is overwhelming that after an initial weight loss phase, most diets prove unsuccessful, they are simply not designed to be stuck to for life.
Of course, the best diet also has to be healthy. Some diets have benefits beyond just weight loss, for example research shows evidence that the Atkins diet can lead to improvement in risk factors for heart disease, hypertension and diabetes, among others. Other diets are neutral health wise, they don't improve it but they don't harm it either, while others are simply not to be followed as they may compromise your health.
Anyone can write a diet and post it online, some wacky scheme to binge at weekends but eat like a mouse on weekdays. But the fact that something is new and flavour of the month doesn't mean it's legitimate; the best diets all have serious scientific research backing their credentials.
Answering what's the best diet is also subjective. It depends why you're dieting. If it's simply to drop a couple of pounds for an imminent holiday, cutting calories might work. If it's because you want to lose weight in a controlled manner and then keep it off something different will be required. What any dieter needs is a diet plan, something that runs through why they're dieting, what their goals are and the time frame. A diet plan is like the map to actually achieving the desired change.
The best diet is one that is healthy, one that leads to long-term change and not just short term benefit. The best diet sets realistic goals. The best diet does not impinge on your lifestyle; you can still eat satisfying meals and you don't have to feel guilty the whole time. And if you apply all those criteria, even to the many tens of thousands of diets out there on Google, you won't find all that many left to choose between.