When clients ask me this question, I usually say that I don't recommend anything lower-quality than Iams or Purina One. Anything you can find at the grocery store is typically more filler than quality food so I don't recommend anything from grocery stores, gas stations, convenience stores, etc.

I can tell you my opinion of best, I can tell you my opinion of the worst, but the key is to teach yourself what to look for in food and have you make an educated decision on what is best for you and your dog. Keep in mind, people will suggest what food they're feeding, and not to say that it isn't good food, but what works for one dog may not work for another. Some dogs' systems can not handle the rich ingredients of super premium foods, eventhough those are typically "the best" to feed your dog.

http://www.dogfoodproject.com/ is a great website to start at.

Good luck!