Food that is fresh and grown locally is always best, since it contains the most nutrients and enzymes. Try planning your diet with foods that are in season and are a myriad of colors.
As a rule of thumb, you want to fill your fridge with foods that spoil. Just think, if an item never goes bad, then it can't be fresh. Foods that have a long shelf-life often have a long list of additives and preservatives in them, too.