Well best is a very subjective term, and I think you'll get different answers from us all on this! And what's best for one cat is not necessarily best for another cat (depending on age, lifestyle, medical conditions etc). Then when you've found what you think is the best, you have to persuade your cat it's the best!
For me, best means wet food with a high meat content, no or minimal grains and no artificial preservatives or colours. Sugar is used as a supplement and foods without sugar are usually complementary (can someone explain to me exactly why sugar is added to cat food please? I know there is a reason - not just to make it sweet!). Peridontal disease is also the major dental problem in cats rather than plaque, so sugar isn't quite as bad as it sounds. It means wet food to me because cats are carnivores and designed to eat meat and not carb heavy dry foods. They also need water. The ideal food, imo, would be raw, or if that's not achievable, cooked homemade. But both of those require a lot of knowledge and care to make sure the diet is balanced so it's easier and safer for most of us to feed commercial food.
Brands I like that meet the above criteria include natures menu, hi life, applaws, almo nature, bozita, cosma and animonda carny.
For dry - the best dry food for me is one with a high meat content and low grain content. I choose Orijen which is a grain free dry food, but I also feed James Wellbeloved and Hi Life dry occasionally.
