I can't decide between Charms or Apparition. Charms appears to be the most useful of the standard courses; I've never quite gotten why Transfiguration is taught at all if wizards are capable of conjuring things out of thin air, except for the Animagus angle. I would think that quite a few effects of potions can be duplicated by charms. And of course the ability to travel long distances instantaneously appeals greatly.
Still to come: 29. Favorite location 30. Whatever tickles your fancy