Every dog is different, so you will know best. I am a big fan of the natura pet food company
The Healthiest Pet Foods in the World – Natural & Organic Pet Food – Natura Pet Products which has a food comparison chart that can help you learn about food ingredients by clicking on each ingredient. Innova is my favorite, its a great balanced diet. EVO is a grainfree high protein diet, some do great on this while it is too high in protein for others. California Natural is a food that many yorkies do great on! It is for dogs with food sensitivities and allergies, keep in mind that food allergies can cause itchiness and dry skin. I also am a big believer in supplementing with a high quality canned food. Just a spoonful or so mixed in with the dry food. Canned food adds moisture back in their diet. Their bodies are like ours in the sense that they are made up primarily of water. It cleans their mouth as they eat. Fresh ingredients are always good for them. Think about it... Do you think you would have the healthiest diet if you just ate the BEST cereal (DRY) you could find on the market with 100% of your recommended vitamins and then drank water?
Good luck!