My favorite stuff personally, especially when the slave knows that they don't have any free will and they develop a fetish for it, enslaving their friends (and possibly family) not because their master wants them to do it but because it's just that hot for them. That element of shameless betrayal is awesome.
My favorites are Tether by Trilby Else and Vitamins by Anonymous-038

mcstories.com/ThaliasHarem/index.html

What I like about this one is that girls who are already brainwashed actually put each other under a few times. In a lot of these stories, the subs are only allowed to use hypnosis on new recruits, so seeing them enjoy some in-house inductions (without even being told to do it in some chapters) is a rare treat.