Lots of if, buts and maybes. I know that wasn't the question, but if ever Angel had not come back, I think Faith could have had the same role in s3, and then she would have taken over the role of Angel in the sense that they would have moved her to her own show. So if Faith has gone bad, I see her staying an outsider and moving away on her own path.
But, had Faith stayed good, I think she would have gotten closer to Buffy. I do think that eventually, they would have developed a close friendship, they are bound too since they are the only two slayers in the world, and Faith has no one else out there for her. I think Faith might have stayed a bit on the outs from Willow and Xander, really like every other person on the show that joined the group later on. Like Angel, or Riley, or Anya, or Tara were never really involved with all three of them (B, W and X). I think Faith would have been closest to Buffy, but would have hung out with the rest of them as per usual for an outside character. She might have grown closer to Giles, but honestly it always seemed like Giles didn't care enough for Faith. He always favored Buffy.
From s3, to me it seems obvious that Faith thought she was closest to Buffy. Even Faith didn't seem to care much for the other characters, but she always seemed to care about Buffy. Didn't matter if it was jealousy, love, friendship, hate. Feelings were always at their strongest between Faith/Buffy.