Objective Power Ranking:
1. Eliza: Great aesthetic, diverse content, easily the girl I've spent the most time on (aside from grinding with Scarlett). The candlestick stories are really good, the crucifix and the dinner table things are also pretty great. She's also got some of the best in-fight content in the game. Literally nothing wrong with her.
2. Lily: Basically what everyone else has already said. Lily stands out from the other girls due to her age, unique character design, and connection with Ethan. The way her narrative is interwoven with the PC's makes her uniquely attractive as a sort of dominant girlfriend type. The fact that she's introduced as a peer rather than an enemy makes losing to her all the more humiliating. The only downsides to Lily are that she doesn't have enough content and her latex outfit is retarded. Wtf did she put on an eyepatch for? First outfit is way cuter. If she gets more content or her second outfit is fixed she might surpass Eliza.
3. Cobra. Best mechanics out of any character in the game. Great character design and personality as well. What makes Cobra especially interesting is her potential; there's so much Bo Wei could implement with her that would make her even better than she already is. There's been talk of an update that will allow for other girls to react to the PC losing levels, which would seriously kick her character and the entire game up a few notches. Its a damn shame that update will never come.
4. Klause: Her name and aesthetic is a little cheesy, but everything else about her is fantastic. I love that you have to be the one to start shit in order for her to start dominating you. She doesn't have enough content though.
5. Scarlett: She's pretty great. Pretty much just a classical dominatrix archetype. She's still attractive enough and the petplay dynamic makes for a serviceable climax to the game's first "arc". Eliza pretty much does everything she does better though.
6. Khulan: I like Khulan and I do occasionally come back to her content, even though I'm not much of a fan of amputation or farting. Which is weird because the whole barbarian from another land thing can be really hot. I've read good Diana the Valkyrie stories before. Maybe it's because Khulan's entire thing is how strong she is and how weak you are, even though she's actually the easiest boss in the entire game. She's the only character I can consistently beat despite being underleveled af. She's also kind of an idiot, which hurts her as a dominant figure.
7. Yvonne: On one hand, she's the most attractive character in the game. On the other hand, that bootlicking minigame is annoying af and I hate it. The footjob stuff is great though, and I like how bitchy she is. I can see why people like her, but that fucking minigame man...
8. Karen: Gaaaaarrrrrbbbbbbaaaaaaaaaggggggggeeeeeeeee