Yuri: Something uncanny about her... not sure what... Can't say I was a huge fan of how sensitive she was and how she overreacted to things... Overall, I'd say she's a good girl... pity she changes the way she does in the 2nd act...
Natsuki: She's not the worst type of tsundere, but not the best either. I get her, but she could get annoying... Overall I tended to take Yuri's side when they fought, but not by much. She's a good girl and stays a good girl throughout, perhaps even improving a bit.
Sayori: I gravitated towards her from the start. Even before finding out about her secret, more so after that. I mean, she's got the most reasons to like Protag-kun so I kind of wanted to help her get with him. Yuri and Natsuki really didn't have too many reasons... Yuri in particular seems to have the least. Monika... well, it's complicated, as y'all know. So, I'd say Sayori is the best girls from act 1, IMHO.
Monika... no sense talking about her in act 1...
I suppose that the best ending would be Protag-kun with Sayori in the machine and Moinka uploaded into an Androdid (Gynoid?)... Poor Monika...