Each of the settlements needs 25,000 Trust Points in order to become Allied.
The easiest by far is Diamond Lake. This is because the rewards for for the missions down south are a lot higher. Also, when you save people down there, they tend to spawn in groups and can reward upto 2-4000Trust per rescue.
Iron Mike's will become your second, as the third act generates more trust missions for it.
My third one was Copelands, getting the last camp job done in the third act and a couple of horde takedowns and I got it.
Obviously, it's easier to get most of the camps to the max level postgame, mainly because the Hordes are so easy to destroy as you can raid every camp of their ammo and explosives before going out.