I don't take sides but Yeonhwa is definitely number one for me. The psychotic side is what make her the best character for me. Dahye is a neurotic and insecure bully but that's what make her adorable to bully.
Dahye commits more crimes than Yeonhwa.
Yeonhwa rapes and emotionally destabilizes girls, but the circumstances of her actions are extenuating, motivated for mental illnesses of psychopathy and multiple personality disorder.
On the other hand, Dahye is a coward that needs to maintain a false sense of strength and superiority when she teams up with her friends to bully people weaker than them. As Ayeon clearly remarked, Dahye doesn't care about friendship, but her own petty interests. Dahye uses people without scruples for her pure convenience, even if it means to make girls vulnerable to rape occurrences.
And no, her sob story of her father pressuring to be a model student doesn't excuse Dahye's vice. Victims of rape don't let other people be raped. Unlike Yeonhwa, Dahye can't allege she has mental illness of any sort. She clearly acts on criminal intent.
The only reason I like Yeonhwa more than Dahye, it's because Yeonhwa IS psychotic. She does manipulate people for her own sadistic pleasures. She wants to posess them. She treats the people she "love" (well, that's a very fucked up kind of love but I guess we can still call it love) like her possessions because she gets obsessed with them, she can't lose them. She hates it. There could be many reasons to that.
But Dahye ? She's a bully. Yeah, maybe she didn't raped (I don't believe Yeonhwa actually "raped" her sister, it seems more like sexual harassment but, my own opinion) people, but Dahye finds excuses. She lives off it.
They both are selfish and fucked up in their own way. But Yeonwha doesn't make excuses. She doesn't seem like knowing what "love" is. She's doing things her way. It's fucked up, but she's genuinely trying to save Ayeon and that other girl. The end is selfish : she's saving them to keep them with her. Dahye just do whatever she wants, then makes excuses to make it look better than it does, letting Sohwa going back to her sister just so she can keep the clip as a "shield" and use it against Yeonhwa.