Who knows. My guess is NCR or Independent/Yes Man.
Usually the best ending, so independent.
Probably a mix that isn't exactly possible as the game is written, like the NCR beats the legion and controls the dam but Vegas retains its independence. Since they probably won't use the setting again then having the NCR push further east will let a familiar faction be present in future games set in unexplored territory (like the BoS - always there, somehow).