Strongest class/job lore wise?

[Shippuu]

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.

Holy used to be 300 potency and the diminishing returns are a recent nerf. Even then, that Death Flare has a 30s cooldown. So 10 Holy's can be put out in the same time (if not more if you want to throw in something like Presence of Mind). So that's 2,000 potency against 400 in the same amount of time. Even with diminishing returns I'd still put my money on WHM~

Even then, if we're counting on potencies, then Warrior takes the cake with the highest potency attack in the game with 500 potency Fell Cleave. Plus 50% more while under berserk. That's one powerful hit.