What is the strongest faction in the fallout universe?

Brotherhood of Steel. They tend to control any area they want to as shown in Fallout 4 they literally fly in and take over Boston, piece by piece (depending on player actions). They are an actual functioning army. They have the best tech, literally they walk around in Power Armour. read more

The East Coast Brotherhood of Steel are without a doubt the strongest faction in the Fallout universe — even if the player sides against them in FO4. Technology; East Coast Brotherhood of Steel (BoS) technology may not be as refined as that of the Institute, but it's all geared for war. read more

The Prydwen in particular was actually built in the intervening years between FO3 and FO4. It wasn't found or anything, the east BoS's scribes built it somehow. A faction capable of expending that amount of resources on a floating mothership of doom has to be powerful as fuck. Not even the NCR has anything approaching that. read more

-_-The most powerful faction in the Fallout universe is the NCR. By New Vegas the NCR has over 700,000 citizens and has enough wealth and weaponry to arm a very large army. The second most powerful faction is the Enclave without a doubt, though they are much better equipped then the NCR they simply cannot compete with it's numbers. read more