It depends on why you want to feed pet food to chickens and for how long.
As a very short term way to get protein into a sick chicken quickly, probably wet cat food is preferable. But neither cat food or dog food is nutritionally suitable for chickens. In view of this, there are lots of alternatives such as ground beef, Vienna sausages, beef liver, fish, and other seafood.
But such high protein foods are not good to feed over the long term as it's hard on chickens' kidneys.