Lore-wise, what is the most powerful job?

[Exodus-E]

When leveling up BLM, I learned that black mages were responsible for a Calamity, and the dark arts are obviously feared across Eorzea because of its sheer destructive power. Heck, the level 70 BLM job quest has you deflecting a meteor that could destroy the entire world.

To be fair, remember that all Calamities are attributable to the Ascians.
Also it wasn't entirely the Mhachi's fault: it was also the Amdapori who heavily abused their magicks (both before and during their war).

But yes, I'd say that both, BLM and WHM would (potentially) be the "strongest" jobs.

Summoner is not as powerful, given it was created with a "niche" purpose in mind.
That said, the potential it can draw upon (like say, out of a really powerful Primal) could make it deal some heavy damage.