Personally from my experience I can name 2 but I could be very wrong about them.

My personal thoughts are 1: the brotherhood of steel Reason being is there in every every game in every wasteland

2: The NCR Reason being is they seem so well organized and have a lot of caravans and traders go thru there territory 3: ???? The enclave, the institute, the legion all have possibilities but from what I know institute doesn't spread far outside the common wealth The enclave cause a a brotherhood member mentions that they've dealt with the enclave back in California and the legion just based on they came from the east made up by slaves but don't think they go far towards the Appalachia wasteland

Feel free to correct and add any information I might have missed keep in mind have only played 50% of the main story 3 and 4 and 100% of the main story in NV amd haven't touched 1, 2, tatics or 76 so there's a chance I'm missing something