@Jethzki Leon seems to get all the heroines and villainesses from each game and Louise simply can't get enough of Leon since the very first time she saw him. I'd rank Louise higher, but her story is in volume 6, which hasn't been translated yet.
Olivia is a great girl, but she's kinda boring compared to all the other characters. She's also almost always overshadowed by Angie in nearly all her scenes and she's basically just there after volume 3. At least Angie still makes her presence known after volume 3. Even Noelle, with all her depressing scenes, still manages to have lots of cute and funny moments with Leon throughout her ordeal.