DEFENSE AGAINST THE DARK ARTS
Since Harry has been at school he has had 4 Defence Against the Dark Art teachers. There was Professor Quirrell (he turned out to be the servant of Lord Voldemort), Professor Lockhart (who lost his memory in the Chamber of Secrets and was useless as a teacher anyway because he talked about himself) and Professor Lupin (who left when everybody found out he was a werewolf). Professor Lupin was the best teacher and he taught them properly about Defence Against the dark Arts with hands-on experience. The 4th teacher of Defence Against the Dark Arts was Mad-eye Moody even though it wasn't the real Mad-eye Moody who actually taught it only at the end of term (see Goblet of Fire for more details). Moody has taught the most dangerous and advanced stuff so far.