I'm sort of a dog food snob. Look at all the ingredients on the foods you are interested in buying. If there are ANY of the following, or even by-products of, DON'T buy it! Corn is a filler, there is no nutritional value to it due to dogs (and pretty much any animal) can not digest it. Companies use it so they can use less of their expensive ingredients and still make a "heavier" bag. It's also a major cause for skin and ear allergies.
What to avoid:
Corn, wheat, soy, animal by-product, by-products in general.
Rice is another filler but it's digestable and has some nutrient content to it. I personally feed my dogs Taste of the Wild. My cat got Merrick but is now also on TotW due to Merrick a bit harder to get ahiold of.