Well I'm not a fan of Sadie, and Susan was too minor and boring for me to choose her.
So it's between Abigail and Bonnie. I preferred the latter of the two and always thought that she could've even worked with John had he been single (Marston did say that she was quite similar to Abigail). They developed the friendship between her and John pretty well imo so I would say she's well-written. Abigail is in both games, however, so even though I don't like her so much, she has quite an important role in both stories so it's hard to really compare them.