@phule The best kibble is the one that works for your pup. I feed my girl Acana, their beef kibbles from their Wholesome Grains line, because it's pretty much the only kibble I readily have access to/never had any stock issues, she loves it, it doesn't contain high legume content (makes her gassy asf), and doesn't have super high fish content (makes her poops super soft).
Purina Pro Plan, Royal Canin, Eukanaba, Hills, and I think some other company too are the ones who meet the WSAVA guidelines, so they're the most recommended. However, each circumstance is different, and it's completely fine if you can't feed one of these kibbles for x reason. What matters is that your dog is happy, healthy, and fed.
There is absolutely nothing wrong with by-products. By-products are the miscellaneous pieces of meats that typically don't wind up on grocery store shelves, such as organ meat, tendons, brains, cartilage, etc., which are some of the most nutrient packed parts of animals. It's like beef liver... Ask anyone around if they like eating liver... Chances are they will say no (I know this because I LOVE liver and everyone I've spoken to thinks it's nasty)... But it's SO NUTRIENT RICH. Liver is so good for you. But people don't want liver when they get their T-Bone steak... So those pieces of meat tends to go towards things like sausages and other things for human consumption, and then pet/other animal food, instead of wasting it.
They just call it by-products because 1) usually these meats are all mixed up together, and 2) most people don't want to read "cow brains" on the ingredient list when feeding their dogs because of societal bias... Same reason why we call them "bully sticks" instead of cow penis.