Like GRL said, there's no best food out there. Different dogs react differently to different foods. To find out if a food is good or not (ingredients-wise), read the label. I only buy the food if a meat is the first ingredient and if the other ingredients look good as well. For instance, here's the list to Chicken Soup for the Dog Lover's Soul (which is a human grade food):

Chicken, turkey, chicken meal, turkey meal, whole grain brown rice, whole grain white rice, oatmeal, potatoes, cracked pearled barley, millet, chicken fat (preserved with mixed tocopherols), duck, salmon, egg product, flaxseed, natural chicken flavor, kelp, potassium chloride, salt, choline chloride, chicory root extract, carrots, peas, apples, dried skim milk, cranberry powder, rosemary extract, parsley flake, potassium chloride, salt, choline chloride, chicory root extract, vitamin E supplement, iron proteinate, zinc proteinate, copper proteinate, ferrous sulfate, zinc sulfate, copper sulfate, potassium iodide, thiamine mononitrate, manganese proteinate, manganous oxide, ascorbic acid, vitamin A supplement, biotin, calcium pantothenate, manganese sulfate, sodium selenite, pyridoxine hydrochloride (vitamin B6), vitamin B12 supplement, menadione dimethylpyrimidinol bisulfite (source of vitamin K activity), riboflavin, vitamin D supplement, folic acid

And here are the ingredients to Iams Active Maturity (feed grade food):

Corn Meal, Chicken By-Product Meal, Ground Whole Grain Sorghum, Fish Meal (source of fish oil), Chicken, Ground Whole Grain Barley, Dried Beet Pulp (sugar removed), Dried Egg Product, Chicken Fat (preserved with Mixed Tocopherols, a source of Vitamin E, and Citric Acid), Natural Chicken Flavor, Brewers Dried Yeast, Potassium Chloride, Salt, Flax Meal, Sodium Hexametaphosphate, Calcium Carbonate, Choline Chloride, DL-Methionine, Ferrous Sulfate, Vitamin E Supplement, Beta-Carotene, Zinc Oxide, Ascorbic Acid, Manganese Sulfate, Copper Sulfate, L-Carnitine, Manganous Oxide, Vitamin A Acetate, Calcium Pantothenate, Biotin, Rosemary Extract, Vitamin B12 Supplement, Thiamine Mononitrate (source of vitamin B1), Niacin, Riboflavin Supplement (source of vitamin B2), Inositol, Pyridoxine Hydrochloride (source of vitamin B6), Vitamin D3 Supplement, Potassium Iodide, Folic Acid, Cobalt Carbonate

You can already see a big difference from the first two ingredients. So when you look for new foods, check the ingredients. Look for meat as the first few ingredients. Some "good foods" that come to mind are:

Canidae, Innova, Chicken Soup for the Dog Lover's Soul (which we're feeding right now), Go! Natural, California Natural, back to basics, Eagle Pack and Wellness.