I use Go Natual and Felidae for my 3 cats.
As mentioned, the first four ingredients are the all important ones. Corn and by/products listed are indications of a less nutriitous food.
Corn is a very common ingredient in pet food - yet often not easily digested by many cats.
Here is a link to a comparison chart.. The foods listed are commonly considered to be "some" of the best available in a commercial food.