My personal opinion.
Divine Seal :
Excellent MP conservation skill, Medica II + Regen + Divine Seal is extremely potent and allows you to NOT heal the boss' Auto Attacks, allowing you to either DPS or /dance to conserve MP.

Cure III :
Very niche skill that I only use in Coil and Ifrit Ex, everyone stacked for raid damage IS NOT a good enough reason to use this skill over Medica II.
Overall Medica II heals for more, and cost lesser than Cure III.
Cure III should be used to PREPARE the raid for raid damage. (AKA, you know shit's gonna go down within the next 5 seconds, Medica II ain't gonna top everyone up in time, that's the time to break out Cure III)

Medica II :
My go to raid damage AoE heal spell, used for 95% of all raid damage, no matter big or small.
Reasoning is even if majority of the regen on the party becomes overheal, the MT who is still getting smacked around is still benefiting from Medica II.
Slapping on Regen in addition to Medica II will more often than not reduce the Cure I spam required on the tank, effectively saving you MP in the long run.

Medica I :
Ironically my personal OH SHIT button for when things are going way south (Redfire blows up in T12, tower explosion in T13 to name a few).
These are unplanned raid damage, so chances are the raid is not stacked on each other to warrant Cure III usage.
No choice but to press 1~2 times of Medicas to handle damage control before trying to stabilize the situation.