Meh. My opinions!
Railroad: Bunch of people who's only goal is to free the synths. No sustainability once the Institute is gone.
Minutemen: Only sustainable with the SS at the General chair. Unless they can get a clear line of succession and government set up, it'll just got the way of Quincy all over again.
Brotherhood: Military force who have a somewhat sustainable goal of purging the wasteland of mutants and synths. While it's debatable on their tactics, the Brotherhood at least won't collapse once their leader dies. They presumably have a way to get a new elder should Maxson die without an heir.
Institute: The bogeyman of the wasteland. While it clearly would survive most of anything the wasteland throws at it, they are hated by nearly everyone unanimously. Sustainable, if they stayed isolationist. If they, however, stopped pulling strings behind the shadows, owned up to replacing people, and furthermore
didn't destroy any civilization as it develops, they could actually become a faction that does what's best for the Commonwealth.
I mean. Bias may be here, but honestly, the Institute does seem like the best choice. They can make gen 2 synths to defend settlements and patrol the wasteland, thus making people more safe and less likely to die doing simple guard duty, they can use their tech to increase the quality of life
everywhere. But they don't, and instead they go for being the Illuminati and just manipulating everyone. I mean, shit. If they just stopped being such a shadow-y cabal, they'd probably be more liked. I'm probably too optimistic though. Seems anytime an isolationist group is around, they have no chance to turn good and always are doing stuff for the 'evulz.'
That was
way more than I thought I'd type up