What is the best cat breed for animal therapy?

It depends on what therapy the cat is going to work most with. Here is the 3rd reason, in the link I've added below, for cats being good therapy animals:

3. Petting or brushing long hair cats can be great physical therapy for individuals with muscle disorders.

Recently, long hair cats have been used with patients suffering from muscle conditions. The act of brushing a cat's thick coat can help to slowly increase flexibility and ease muscle and joint stiffness. Not only is brushing a cat's hair good for exercising certain muscles, it is also quite relaxing. The best type of cat for this type of pet therapy is said to be the long hair Persian breed.

http://www.perfectcatcare.com/cat-care/cat-therapy-%e2%80%93-reasons-why-cats-make-good-therapy-animals/