In the FFXIV world, lore-wise which job is the strongest if there was a tourney?

[Shippuu]

Estinien on the other hand is strong because he's a Dragoon, period.

Except he absolutely isnt. He was the azure dragoon, his higher powers coming directly from an eye of nidhogg, and even with that he played second fiddle to the WoL in the 50 drg questline. After that in heavensward he continues to use the eye of nidhogg for power, gets possessed and basically merges with nidhogg, and in stormblood we see he is capable of harnessing that power still, though upon destroying the eyes for good it may severely diminish his abilities.

He's still absolutely an exception and his heightened power is not because he's simply a dragoon, but because he is given (or takes) power from a being much stronger than himself. He's owes his strength to an outside source just as much as that blm NPC you just dismissed.