Elder Scrolls via specialized weaponry. Yeah, one BoS platoon might be worth an entire Imperial Army, but the TES have all of the Daedra gods, summoning, the Morag Tong and Dark Brotherhood, and a buttload of dragons. The morale factor would be pretty big, though: a lot of Fallout's world is already sort of mentally broken. How many do you think are going to stand and fight when they see a wing of dragons swooping down? On the other hand, if Fallout can draw TES into open battle and utilize the huge firepower advantage, it would be crushing. Meh. I got TES by a thread.

Elder Scrolls. Fallout is obliterated (already).

The Deadric gods alone could wipe them out. Plus, we got Auriel's bow that can block out the sun or can send violent solar flares, plus many more weapons.

Gods vs Nukes. Gods win.