Of the options so far, Saiyuki Senpai is far and above the best. Drop dead gorgeous, check. Devoted, Check. Into kinky stuff, Triple Check.
Yuika? Oppressive, lacks the beauty of Saiyuki (she's cute, yes, not denying that), and is a sadist, that ends it for me.
Nanjou? The Fujoshi theory going around would certainly end this ship on arrival, but also think that she has had this much time to do/say something and hasn't, even as two other girls are making clear and aggressive moves on him.
Imouto? I ain't touching that. Yandere, incest, I don't know. There is a whole lot of gone you wanna put between you and a woman like that. Goodbye and good luck.
So yeah the ranking then are: 1) Saiyuki [winner by a landslide], 2) Yuika [by virtue of most exposure after Saiyuki], 3) Nanjou [cause there is something wrong there, we just don't know what yet], and tied 3) Imouto [that is one fuckbasket you want to leave alone right now].