The best diet for a species is one which most nearly approximates a natural diet for that species. In the case of humans, the natural diet is one free of hormones and pesticides, free of refined foods, especially sugar in its many nefarious forms (white sugar, corn syrup, concentrated fructose, etc). For more information on diet for people, pick up some of the literature available at your local health food store. Or do a search on natural diets. Go to Dr. Mercola's excellent website, and sign up for his newsletter. Look at the library for books on eating naturally, or eating for your blood type. Eat lots of fresh fruits and vegetables; whenever possible, go organic and/or eat local!
For dogs and cats, the diet which most mimics the natural diet is one of raw meat and raw meaty bones, with some green vegetables. For more information regarding diets, see Dr. Pitcairn's book, Complete Guide to Natural Health for Dogs and Cats, and visit these links for raw feeding for cats: www.catnutrition.org and www.rawfedcats.org and for dogs: www.barfworld.com and www.rawlearning.com.
For much excellent information on many aspects of diet/vaccination/healing, visit: www.shirleys-wellness-cafe.com