I've had good luck with Taste of the wild.
And also Merrick, if their stomach can handle the higher protein.
I've had good luck with Taste of the Wild too. Any flavor. ALSO...recently I had discussed here about impacted anal sacs so I did some research and a very high recommendation was canned pumpkin for fiber to increase size of stool. My God this stuff is amazing! She LOVES it added to her food and her poops grew 3 times that day! lol Haven't had to have her anal sacs expressed since <fingers crossed>
You may want to add fiber and pumpkin is terrific. The brand I use is Nummy-Tum-Tum Organic Pumpkin. Can't recommend enough. Good luck!