Any high quality small breed kibble is good. Purina makes one just for Chihuahuas. Science Diet Small Bites is an excellent choice. Also try Science Diet T/D to help prevent dental disease which is common in chihuahuas. If you want your dog to have the best, then make sure you focus on dental health when you purchase a food.
Most stores carry pet foods for small breed dogs. The pieces are made extra small so your dog can chew them and it's formulated for the health of a small breed dog.
There are plenty of high quality small dog foods in your local pet store. If you think your dog is too good for pet store brands then talk to your vet for advice. My suggestion though is to treat your dog like a dog, caesar milan has great ideas.
There are foods marketed for chihuahuas, such as Royal Canin, but these are not great foods. The best foods are grain free, like Taste of the Wild or Wellness Core.
The best "special" foods you can get is the ones you make yourself. Sweet potatoes, rice, any veggies, ground fresh meat, NO ONIONS OR GARLIC! Otherwise, try Caeser, Iams, Beneficial.
They have special dog foods that are made just for Chihuahuas, you can get them at Petco or some other animal supplier. It's made with smaller bits so that it's easier for your dog to eat. They also make smaller treats for Chihuahuas too. You may want to check with your vet for more suggestions.
