Out of Tamriel's long history, which army or military presence has had the biggest impact on the continent/ which is generally the most formidable and powerful? I don't mean anything like the Numidium or such, because that's a giant machine, not legions of living beings.

Also, not a Daedric army, as that would be literally innumerable. Mortal army in mortal history. Which was the most powerful?

Toss-up between the Imperial Legion and Ysgramor's 500 Companions.