It is my belief that in a duel, the most useful spell in a witch or wizard's arsenal is the shield charm (protego). One cannot be reliably expected to attack if one is incapacitated, and the shield charm allows the duelist to protect themselves from the efforts of their opponent. Some might argue that a shield charm is not flashy, however "flash" does not win competitions—success does. It also helps to prepare duelists for actual combat, should they be unfortunate enough to find themselves attacked in an uncontrolled environment.