I don't know very much about any of the characters aside from Flandre and Cirno to some extent. I do have a thing for insane characters, and she seems to be pretty unstable, so she'd likely be my favorite anyway. I also have a thing for diamond motifs.
Gonna try to get Hanako's storyline going here. So far Emi is my third favorite character after Rin and Kenji, but I'm gonna do Hanako first since she's mysterious and my friend hates her for some reason, I'm rather curious to learn more about her.