What is the best food for my Shih Tzu w/ skin allergies?

Are you sure that she's just allergic to a food ingredient? If so talk to your vet about feeding a food elimination diet. This way you can maybe find the food that she is allergic to and can keep it out of her diet. You must be careful not to feed her anything else, no, treats, no rawhide, no real bones, no flavored tablets, nothing except the food. Keep in mind that your dog can also have allergies to weeks, trees, gr, and allergens in the home including dust mites and tobacco smoke.