I'd say mostly>d) be reduced to being a monster/beast in the villains' forces
It's best when they turn into a slavering fuck-puppet or part of a meat-wall. Imo that's the furthest corruption because they completely forget about everything but pleasure. I like certain things for all of them.>c) be reduced to just being a footsoldier in the villains forces
This is my second favorite, but I really only like it when there's identity loss-- when the heroine becomes indistinguishable from any nameless footsoldier.>a) become an equivalent high-tier villain
There's a lot of this, where the magical girl gets transformed from "Pretty Curator Cindy" to "Poisonous Curator Cindy" and betrays her friends. It's kinda meh imo, relative to the other options.>b) go full ham and supplant the villain
This one might be the most interesting but I can't recall ever seeing this in hentai. Closest I think I've seen is when one magical girl defects to evil, evil allows her to bait in the other magical girls, and then once the good once are captured the evil monsters turn on the traitor magical girl as well.