Strongest class/job lore wise?

[AspectOfWinter]

Let's see:

Holy - 200 potency AoE that diminishes with each target hit
Flare - 260 potency AoE that didn't originally diminish, but now does
DEATH Flare - 400 potency AoE that does not diminish at all

I think that answers your question.

That, and it's implied in the level 30-50 SMN quests that the Egis are actually MUCH more powerful lore-wise than they are gameplay-wise.

As for WHMs and BLMs bringing about an Umbral Era, wasn't it the end result of hundreds of mages recklessly using it for warfare over many years?