Tiering, huh? Based off of my experience on Aether, I would probably rank classes like this:

S:
Paladin*, Warrior

A:
Astrologian, Scholar, Summoner, Black Mage, Ninja

B:
White Mage, Machinist, Dragoon, Bard

C:
Paladin, Monk, Dark Knight

Paladin is incredibly powerful. They provide the best utility of any class, do respectable damage, and are amazing at keeping their party alive even relative to a warrior thanks to Testudo. You only really need 1 or 2, though, and the rest are not very good at all.

On healers:

Scholar has the highest skill ceiling of the 3 healers, and the best single-target burst. Its only real weakness is relatively inefficent sustained healing, but Scholar's incredibly powerful OCGD heals carry it in bursty encounters.

White Mage has the highest potential AoE healing, but has trouble when focused. Their single target healing is okay, but can't really match a Scholar turreting with Dispersion.

Astrologian has a very high skill floor, but doesn't have the tools to excel at turret-style healing - Synastry is good, but doesn't match the power of Adlo+Lustrate, and they can't hold a candle to WHM AoE.