Solid Gold Wee Bits is a very good dry food. Also Wellness products have good ingredients. The ingredients they put in the cheaper brands like Purina is basically road kill, leftover stuff from the slaughterhouse, like beaks, hooves, eyeballs and other such junk, plus a lot of filler that dogs can't actually digest and use because dogs don't need a lot of grains and that what they put into the food to cut costs. Also some of the meat is from rendering plants meaning they take the cattle,pigs, horses that have died due to diseases, any other dead animals that the plant is called to pick up, such as dead animals put down at the vet.
So it's best to look for dog food that contains real meats (the first ingredient shouldn't be meat by-products or grains). Check out this website for a guide to picking a good dog food for your pet:
http://www.wikihow.com/Choose-Healthy-Dog-Food