You are unequaled at the casting of one particular spell.
Prerequisites: Spellcraft 15 ranks, at least three metamagic feats.
Pick one spell which you have the ability to cast. Whenever you cast that spell you may apply any one metamagic feat you have to that spell without affecting its MP cost, as long as the total modified cost of the spell does not exceed the maximum amount of MP you may spend on a single spell. (For example as a level 17 Black Mage you cannot add Quicken to a 9th level spell as its effective level would become 13)
In addition, if you have other feats which allow you to apply a set numerical bonus to any aspect of this spell (such as Spell Focus, Spell Penetration, Weapon Focus [ray], and so on), double the bonus granted by that feat when applied to this spell.