I think some are just really biased
In terms of prettiness they are all pretty, I mean its a Bishōjo (beautifull young girls, according to the wiki) VN so ofcourse they are all going to be pretty, the only difference is who you find prettier than the rest. Now on to my ranking and I will try to be as less biased as possible.
Pretty long post with my ranking and explanation so beware heheh.
Prettiness:
Emi: I guess I like the petite type of girls, she is beautifull and I really like her playfullness too and wish more girls would tease and flirt with you because they love you. I would so hug her.
Lilly: Like others have said she is the "traditional" beautifull girl but I really liked her ingame sprite. Also have a thing for blondes.
Hanako: I really like the long hair and her adorable smile. Also love her city outfit look, so pretty.
Shizune: Thigh highs, she really pulls that off and the short bobbed hair she has is also very pretty and suits her. But im not into glasses so much so meh.
Rin: I like red hairs too but for some reason altough pretty I'm not that attracted to her, dunno why exactly, maybe its the sprites like others said.
Sexiness:
Shizune: Again thigh highs

no contest there.
Lilly: She got all the right curves and is very beautifull and has the... ahem healty adolescent sex drive.
Emi: I really wish she would let her hair down more cuz in that last scene, she was tha bomb.
Hanako: Altough she has a nice body and the right curves as hisao might say, I cant seem to recall a moment where I thought "damn shes sexy", more like "wow shes cute".
Rin: Well no suprises here I guess, dont really find her sexy but pretty instead.
PS: I forgot that tagging spoilers only blacked out the text, kinda wish it folded them so you had to click the unfold button to read, if that was the case I wouldve done that.