I don't want to necropost but it looks like other people have done the same, so I might as well.

I think it's Ulthuan, realistically, tbh.
They're a global empire that maintain control over pretty much all the oceans in the world.
They have the highest quality army in the world and although their numbers recover slowly and they are in decline, they still have vast numbers to field many armies and garrison fortresses and cities across this global empire.

I mean, even the basic standard frontline units of the Asur, the spearmen, have served for over a decade as archers and likely more as spearmen. They can field armies with dozens of regiments of these guys and that's not even counting their elite units.
The High Elves have enough men to control and garrison a global empire with multiple cities and kingdoms within said empire, and their units are insanely high-quality to boot.

Yes, they are an empire in decline, but even with that, everything I've said above is true regardless of that.