Which is the most powerful role?

[Umero]

Tank is obviously the most powerful role. It has the least waiting time in Duty Finder. From sketch, leveling a tank without any friends help would be way faster than leveling any other job.
So please play tanks more, guys! Help the game out! We need more tanks!

[SeraviEdalborez]

Virtually everything that matters resists every kind of CC except sometimes Stun.

[TheTen]

In low levels, I would say healer, because even without a tank and mediocre DPS, a healer can carry a party through everything. The higher the levels, the more important the tanks becomes, and eventually, you get into most fights being DPS races, at which point all three roles are equally important to the group's success.

But: Healer undoubtedly feels the most useful, because you get constant immediate, crystal-clear feedback when you 'do your thing'... HP bars going back up. (And dead people reviving.) As a tank or DPS, your contribution only really becomes apparent once the enemy is defeated.

[Judge_Xero]

My list would be
SCH - WHM - Tank - SMN

A good healer can carry a party the best. SCH has the best enmity management and burst heals so ideal to carry the party. WHM is a great healer but in burst situations can end up pulling hate from a bad tank.
Tanks can carry a lot of the party but if they don't get good heals they can only mitigate so much.
SMN is the best DD to help carry a party. Battle Raise. Pet Tank. Party Buffs. Enemy debuffs. Crowd control. Great kiting between Ruin II and Sprint.