Superlative Healing is a skill in the Heavenly Light skill tree of the Mage class.

Skill Description: Improved healing spell from the advanced one. Heal the HP by x. It has independent cool time.
Required job: Third Job
Required skill(s): Advanced Healing
Required points: 45 points in Heavenly Light
Is requirement for: Healing Mastery
Skill type: Support Skill
Attribute: Light
Cool time: 12 seconds
MP Usage: 175 MP
Target: Any Player
LevelHP recovered
1 9750
2 13000

The strongest heal you can get. Probably overdoing it a little bit, but the fact it has a separate cooldown from the other heals suggest it's probably worth getting.