A question right up my alley...
Read these links, then go from there.
http://recipes4gourmetdogs.com/2011/08/worst-dog-food-brands/
http://www.dogfoodproject.com/index.php?page=betterproducts
http://members.petfinder.com/~CA469/files/DogFoodRatings.pdf
Corn is the worst...
http://www.rchumanesociety.org/_files/How_Diet_Affects_Behavior.pdf
I have met trained and rehabilitated many dogs that had specific behavioral problems that were made worse by corn product in their feed.
Ideally, you want to read the ingredients list. Just like with human food, the ingredients at the top of the list comprise the greatest proportion of content. You want to start with a REAL protien. Chicken, Bison, Beef, etc. Stay away from protein meals (chicken meal, chicken by product meal for example) as a first ingredient. These are somewhat acceptable farther down the list of ingredients, but not at the top. Stay away from corn products. Some dogs have wheat, soy, or white potato allergies as well.
Stay away from dog foods that do not list the caloric rating per cup of food. Buy a dog food with a caloric rating, then use this table
http://www.dogfoodadvisor.com/dog-feeding-tips/dog-food-calculator/
to determine the proper amount of food to feed your dog...
We do rescue, boarding, boarding training, and long term medical boarding (long term illness/injury rehab). We typically carry 7-9 different brands/types of dog foods depending on the needs of our clients/boarders.
A few of our favorite dog foods
Taste of the Wild Bison Formula: A high protein food suitable for high activity level dogs, and hybrids. This may be to rich for many low to mid level energy domesticated dogs...
Blue Buffalo Wilderness: an excellent food for most dogs, again this is a high protein food and may be to rich for some dogs
Solid Gold and Canidae are great as well.
For some medical fosters with sensitive stomaches, Chicken Soup for the Pet Lovers Soul Light formula.
Hope this helps...
Sorry for the long post