I like both girls for different reasons. Linda's and Banri's interactions feel more genuine and real, even though Linda has chosen to create some distance from Banri. And that's another reason I like her. She knows that bringing up the past might complicate things for Banri, so she gets bonus points for being so caring and selfless.
I didn't like Koko at first, but she's growing on me. It's very cute to see her try to be a "proper girlfriend" to Banri
But as of recent, I feel like their relationship isn't really progressing and going anywhere. And the reason is that stupid Banri Ghost longing for Linda and it's hindering Banri from growing closer to Koko. It' so annoying D:
So in the end if I had to choose, I prefer Banri and Koko together. I still like Linda, but she's in the past and Banri should stop dwelling in the past and move on.
Best girl is still Nana though.