I've been wondering about the same thing. I have been feeding my little chihuahua "Pet Promise." It claims to not have any glutens and preservatives in it..suppose to be all natural. I have heard that "Wellness" is excellent. I'm thinking of changing.
There are many brands out there. Good ones will have NO gluten and NO grains (this includes rice and corn), a good protein source (meat, fish, eggs, tofu, or poultry), green leafy vegetables, sweet potato, berries and some other fruit, fish oil (containing DHA), vitamins, minerals, probiotics, and digestive enzymes.
The good food does not have to every single ingredient that I have listed, but will contain many of the ingredients.
Some good brands: Wellness Grain Free, Evo/Innova, Orijin canned only.
Please feed mostly canned, since this is best for the kidneys and to prevent subclinical dehydration. Dry food does little to prevent tooth decay. Tooth decay is dependent on heredity and dental hygiene such as daily brushing of your dog's teeth.
Even the best food may require additional supplements. For example, if you dogs' coats appear dull, add extra fish oil supplements.