Separate names with a comma.
My heart says Legion but my rational brain knows that’s just faction loyalty speaking. ;-;
Wanna know what the best faction is? The one that gets hardly any love. The Tunnel Snakes. Tunnel Snakes rule! Ha ha, but seriously... hard to say. Maybe the Brotherhood if they hadn't been butchered by Bethesda.
The Master's Super Mutants. We see their rise and fall, we meet their leader, their rank and file, their fanatic followers. We learn their almost mythological origins (I like that they've become this strange detail in the origins of the NCR and Enclave and other groups).
Obviously Little Lamplight.
To not repeat- Winds of War from Fallout Nevada.
I think the Master's Super Mutants is a solid answer. Legion's worst enemy was development time constraints Aure haha. I also can't speak on Nevada or 1.5