I think that's like asking what is the best food for humans - there's no one right answer. As has been pointed out, cats are as individual as humans; what works for one might not work for another. Years ago, I had two cats, both adopted as kittens only a year apart; both had the same access to the same food - grocery-store food was all that was available back then. Cassandra lived to nearly 17, and Kalli made it almost to 20. That proves to me that someone who posted on another site that "grocery store food will kill your cat" wasn't really right.

A few years ago, I did change my current cats' food to grain-free (Taste of the Wild), and I have noticed that Mordred is a bit more active; however, he did NOT lose weight on it, which some people claim will happen. I also feed him some Dave's canned cat food (half a small can) in the evening; Althea gets a quarter of a tuna-sized can of Merrick's, and the outdoor cats (Smokey and Shadow) split a can of Fancy Feast.

My advice is to just do the best you can. Any food that has the AAFCO label has all the nutrients a cat needs; the difference in price reflects that fact that the higher-priced food has higher quality ingredients.