Choosing a dog food: what to look for
This is an excellent guide to the terms used on bags and tins of dog food and will help guide you to better choices (you can print it out and bring it along to the supply store).
Keep in mind that after these basics, choosing a food is pretty much a personal choice and there is no such thing as a 'best food' -- much depends on your personal preferences and the individual dog. Dogs like variety and generally many recommend rotating foods (not daily or weekly, which can imbalance the diet, but after one bag is done perhaps try a different food or flavour).
Hint: most dogs care far less about the brand of their food than the owner does, and do not care about trendy expensive ingredients. Recall that few dogs in the wild ingest some of the more exotic ingredients likely to appeal to their owners. If you are happy to pay a premium price for some of the foods out there on the basis of such ingredients, just keep in mind you are doing it for you, more than the dog. :)