Hi, usually a supermarket just doesn't offer the premium quality foods. I personally no longer like to feed Iams, but some do and some feel it is a quality food - that would be the one possible exception since this is now sold in a supermarket.
I also feed some of the canned foods I find at my local market, because Fancy Feast helps re the hit or miss appetite of my kidney failure cat & gets him eating, and bottom line is he must eat.
The general advice that I give, is to buy the best quality food you can sensibly afford, choosing from brands that do not use corn gluten meal as their only protein source or as their first listed ingredient, looking for ingredients lists that have real meat/chicken/turkey or chicken meal as the first 3 ingredients. Avoid if possible, foods that contain ground/whole or cracked corn, that use multiple carb sources in the same food such as wheat and soy flour, corn, and/or rice/barley etc. Having one such as rice in the recipe is one thing, but having 3 or 4 is quite another.
Avoid foods that use BHA/BHT as a preservative, or have artificial flavors added or dyes/colorings.
Some brand names to consider - PetGuard, California Natural Chicken and Rice, Innova, Felidae, Nutro's Natural Choice Complete Care, Natural Balance, and for a decent food but in a lower price range than the foods above - PetSmart's own Authority brand.