As a White Mage in Final Fantasy 14: A Realm Reborn, I respectfully disagree with all choices mentioned. (Except the first one for obvious LoK reasons)
Full-Life. It does what the name says. In Final Fantasy 14: ARR, this is called Benediction, where it restores ALL of your Hit Points, no matter what the maximum number is. If you manage to die in my party on my watch, or several others as a Full Party, then my level 3 Limit Break "Pulse of Life" will revive you instantaneously and with full HP and MP.
<------ White Mage for life. Especially if Miyenne and IndomitableSam are in my party.
Picture Might and Magic 7. You decided to be an evil jerk, then killed yourself (became a Lich), now what? One word: Souldrinker. Souldrinker is easily one of the best spells in the game. You cast the spell, the screen goes black from your dark conjurings, and suddenly everything around you is either dead or severely damaged and you are all healed up. Not just yourself, mind, Souldrinker's life-stealing power is so strong it actually heals everyone around you as well. It's got pretty cheap mana cost and has a relatively quick cool down for a high-damage AoE spell (about as spamnmable as a basic Fireball), and pretty much the only reason to go into Dark Magic.
The best part is, it doesn't matter what you cast Souldrinker on: demons, elementals, rock creatures, (magic-immune) dragons,... Chow down on that delicious life essence, you evil bastard... at least until you get the blaster guns, then everything else is pretty much useless.
PS: Yes, I know Dark Priest can use Souldrinker too, but they just don't have the mana pool to spam it, and they're better for buff spells and raw heals anyway.