This is probably one of the most dreaded question for a dietitian. As soon as I tell new friends about my occupation, I would be asked to recommended the best diet, the best food or simply asked what myself eat!
If you are so inclined to read the full article (1) here, David Katz and colleague from Yale University published a good article titled 'Can We Say What Diet Is Best for Health?' in the Annual Review of Public Health. The article has a good summary of different 'diets' style - low fat, low carbohydrate, vegan, Mediterranean, Paleo and Mixed balanced (based on National Dietary Guideline - it is an American paper).
Instead of summarizing and rewriting the excellent article, this is the direct quote from the Discussion section.
Can we say what diet is best for health? If diet denotes a very specific set of rigid principles, then even this necessarily limited representation of a vast literature is more than sufficient to answer with a decisive no. If, however, by diet we mean a more general dietary pattern, a less rigid set of guiding principles, the answer reverts to an equally decisive yes.
The aggregation of evidence in support of (a) diets comprising preferentially minimally processed foods direct from nature and food made up of such ingredients, (b) diets comprising mostly plants, and (c) diets in which animal foods are themselves the products, directly or ultimately, of pure plant foods—the composition of animal flesh and milk is as much influenced by diet as we are —is noteworthy for its breadth, depth, diversity of methods, and consistency of findings. The case that we should, indeed, eat true food, mostly plants, is all but incontrovertible. Perhaps fortuitously, this same dietary theme offers considerable advantages to other species, the environment around us, and even the ecology within us.
So in short, there isn't one true diet. What we are aiming for is a eating style that comprises of food sourced directly from nature and foods made of those ingredients, i.e. minimally processed and mainly from plants. There is no one food or one nutrient for that matter that is the BEST of all.
Next time if someone asked me the same question, I may have to bring this article along.