The technique I consider to be the greatest is Makarov Dreyar's Fairy Law. A legendary spell in the Fairy Tail world as well as one of the "Three Great Spells" of the Fairy Tail guild.
When in use Fairy Law slays any darkness, the spell only affects those whom the castor sees as enemies. Which is a really good bonus when using a a technique of this caliber; usually techniques with a lot of power can hurt not only the target but also innocent bystanders. @VinMcCarthy