I think saying there is no ONE diet for all, is a cop-out. Ideally humans should be eating what their physiology is designed for; and that is no animal products or other processed food. Our intestinal tract is unlike any carnivores, and this applies to all humans, we are most similar to frugivores. Also, we are the only species that cooks food allowing us to eat plants that we would not be able to digest.

This is not even mentioning the fact that humans enslave animals for food and their own pleasure at the cost of so much in this planet.