As long as it is treated seriously, parent-child is the one that is most interesting to me. Though this assumes the child is underage, and not a consenting adult, so I guess it doesn't fit into the options on the poll.
Specifically, I like reading this kind of the story when the child is romantically in love with their parent, while the parent is like "Okay, wtf, my child loves me romantically, what the hell do I do now? How do I stop this?"
It gets particularly more interesting when the parent starts actually having some romantic feelings for the child and goes, "Wth wth wth!? What am I feeling!? Wtf!? How do I throw those feelings away!? How do I stop this from getting any worse!?" and then starts seeking out help from friends and/or a professional to like... Get help, because they know that what they're feeling is totally messed up and isn't right at all.
This is the kind of incest I enjoy reading about. The one where at least one of the parties is incredibly aware of how messy everything is and desperately tries to find a way to not mess up, and like... Actually tries seeking out help.
It's just interesting to see it being portrayed like that, yanno? The incest there doesn't exist to give a "sense of forbidden" to the story, but it is instead a core theme that is treated seriously as something messed up. As a problem that the adult in the relationship really needs to solve somehow, even if they actually reciprocate the feelings of their child.
