Which fictional school subject does J.K. Rowling identify as Harry Potter’s favorite?

Harry Potter’s favorite school subject is Defence Against the Dark Arts.

The subject teaches students how to recognize and resist dark creatures, curses, and other forms of dark magic. It is one of the core classes at Hogwarts and becomes especially important because Harry’s life is repeatedly affected by Lord Voldemort and dark wizards.

Harry performs particularly well in the subject, although the teaching staff changes unusually often. Professors Quirinus Quirrell, Gilderoy Lockhart, Remus Lupin, Alastor Moody, Dolores Umbridge, and Severus Snape each teach the class during different periods described in the books. Rowling has linked this instability to the idea that Voldemort cursed the teaching position.

The subject is often confused with Charms or Transfiguration, both of which Harry also studies. Defence Against the Dark Arts is the one specifically associated with his strongest interest and practical confidence.