It is easy to get overwhelmed at a pet store with the many brands of food now available and the intense and aggressive marketing by some companies! First and foremost, DO NOT get overwhelmed or offended by words like BY-PRODUCTS, this a term gets a bad rap! By-products are simply parts of the animal that humans don't want to consume eg sometimes this means the intestines etc and other parts of the animal that are still high quality and provide plenty of nutrients to your pet. Also avoid grain free, exotic or "boutique" diets unless you consult your veterinarian.
But generally, I stick to Royal Canin and Hills BOTH of these companies have put EXTENSIVE research into their diet formulations so if your pet only ever eats one of their products, they should be getting all nutrition that they need to live a happy healthy life