hmm, i think it would be a close tie between dystopia au amon and idol au kidnapper

they're evil in quite different ways though.

amon is very self aware and he knows he is a selfish asshole, he just doesn't care. he can effectively put on a mask and pretend to not be as bad but he sees other people entirely as beneath him. he knows that him doing what he does to sol is morally wrong but he has along accepted it and he doesn't care about that.

amon is extremely evil in his actions but as a (fictional) person I cant really hate him because at least he is honest. i have a weak spot for awful characters that are fully aware of how bad they are, i admit XD

he is lowkey inspired in kanou from okane ga nai, but unlike kanou who mostly >tries< to avoid hurting ayase directly, amon actively gets off from hurting sol. he just gets really mad when ppl do it without his express authorization :p

kidnapper, on the other hand, was created to be not only evil but someone i would truly hate (even being in a fictional setting).

so yea, personally i do think he is worse.

he is the opposite of amon in the the sense that he truly believes himself to be a good person
he truly believes he is saving sol's life. he is so good at justifying his actions to himself that he really thinks that sol's "punishments" are something he only does out of necessity, for sol's own good, and not because he actively gets off to that.

in his mind, his extreme bigotry is not only justified, but logical. he isn't wrong, its the rest of society that is.

and, as i said before, he truly believes he is saving sol from that "fucked up world" as he sees it

i think that's what makes him such an infuriating character

another, uhm... interesting(?) thing about kidnapper is that he is actually kind of a good father to their kids. kiiiinda. he doesn't really do any of the actual work that raising a child involves, but he does try to spend time with them and be somewhat present. the kids genuinely like spending time with him and run to the door to say hi when gets home. basically he gets to be the "fun parent" and poor sol gets stuck with being the "nagging, annoying one" since he actually has to educate them.

which yes, is very fucked up.

anyway, he is a good parent to children and only children, once they turn into teenagers and start wanting some autonomy, shit starts going bad :p now suddenly sol is the good, reasonable parent in their eyes lol. well also because i assume teenagers are more aware of shit happenning around them than you'd expect. I'll stop talking tho, don't want to spoil the plot I have mind :p

anyway...

there's two other characters that could in theory compete with these two in awfulness which are ganenver's ex fiance and drover (from lie) but i haven't developed them that much yet so just know that the first one is more similar to kidnapper and the other acts in similar ways to dystopia au amon (but without the mafia boss type of money and influence or living in a dystopian society lol)

i don't think so, tbh. amon, at the start of the story is still very stubborn and set in his own ways, which means that while he disagrees with people that treat pets as literal objects and think they should be treated "better" (his own idea of good treatment is quite messed up though, considering his job), he ultimately still believes in the pet system. he would likely think sol is just "being silly" and at most might try to accommodate for physical dysphoria if he sees sol is too depressed but would still treat him as an angel/not take him seriously as a demon

could it change as the story went? yea, probably, maybe even faster than with canon sol but sol himself would end up with much, much more resentment and anger towards him

the ocs i have that mostly aligns with what you asked you be either lily, dagon and hael, in order lol

lily not so much actually because she's transfem in a more "close to irl" way, since she's trans in the gender sense, but i thought i'd still mention her because you asked about procedures and she does take a in-world equivalent to hrt and has had tail reduction surgery :3

dagon doesn't really see himself as either angel or demon, but he's also not suuuuuper dysphoric about it so he never seeked anything procedure-wise in that regard. except for boob removal LOL. the little dysphoria he still has is about his hooves and while there is surgery to make them look more like feet, most doctors would refuse to do it without tail removal (the excuse is balancing issues) and his tail is VERY important to him, so he wouldn't do it. he might shave his horns from time to time (horns in inferna work kinda like nails except with nerve endings :p) but he wouldn't be worried about keeping them that way, he'd enjoy the different stages of growth and then shave them again once they reach max size lol.

also there's hael, who is a trans demon buuuuuuut ended up drafted. he also ended up being bought by dagon, who actually believes that the draft made a mistake with him and tries his best to be accommodating and let him live as a regular person in secret and without getting either of them in trouble. dagon eventually manages to get him hrt-equivalent meds that would make more hair grow on the end of his legs, and make horns, tail and ears grow :p. For ppl assigned angel at birth(?), however, due to their regen, they need to remove those parts continuously and wait for regrowth each time, while taking hrt, to see effects. (this is a bit hard to explain, hopefully it makes sense?) i think he'd only feel safe enough to get surgery for either wing removal or feet reshaping once things start getting more chaotic in inferna and the pet system start to crumble for real, so it would take time.

(if anything about hael doesn't make that much sense, read his page https://s4dsqu1d.nfshost.com/inferna/canonprofiles.html , it should help, i think)

also to answer some more about procedures specifically:
-there's hrt for "both sides" that work on horns, tail, ears and certain places of body hair development, ether by slowing down/shrinking or making it grow. for growth, you need to have regen powers + keep cutting the limb off
-they're pretty advanced medically so most surgeries you can think of probably exist.
-ppl assigned angel at birth usually have regen powers and ppl assigned demon at birth tend to have enchanting-like powers, that's the only thing you can't really change :s
-well, technically you can't change height too drastically either, but you can change it a bit to make dysphoria related to it more manageable