What food should I be feeding my cat?

When my cat was a year old, he started developing crystals in his urine. The vet put him on a special diet of Feline C/D. We had no more problems for six years. Unfortunatly, he has just developed an allergy to his food. It caused him to break out in crusty scabs. The vet gave us Prednisone to clear it up, and now he seems fine. But the vet told us that if it happens again, we'll have to change his diet. I am trying to get as informed as possible BEFORE this problem happens again. Does anyone know of any other good quality foods we can try him on that prevent urinary crystals, just in case? Thanks!