I've done a lot of research on this lately too - you can check out my post... I've found, IMO, the best value for the money is Preformatrin Ultra.. I actually just bought a bag last night - so I can't tell you any difference I see in my dog yet but the ingredients look great - no corn at all, all natural food.
adited to add:
here are some good links that helped me on my search
http://www.api4animals.org/689.htm
http://www.thelittlefoxes.net/html/pet_food.html