If Maya weren't in the game, I'd chose Josy in a heartbeat. If Josy weren't in the game, I'd happily pick Maya and see if she and the MC could make it work.
But since both are in the game and I can only choose 1, I'm really not sure. Prior to Episode 6 I'd have said Jill was a fine choice, but I can't take her seriously as an adult right now. Sage would make a great friend, but I think she's looking for something very different in a relationship than my (main) MC - and we know she handles that very badly. Bella is a non-starter until she works her way through whatever the hell happened to her. There are a few other girls that have promise, but they seem either unable or unwilling to start a serious relationship with the MC.
So I guess if I had to choose, my MC would pick no one, support Maya and Josy as a couple, and hope that either Jill drops the idiot ball or a nice transfer student shows up in Season 3.
The problem is that Sage couldn't accept that the relationship was history. She convinced herself that it was still salvageable despite Chad all but ignoring her. Worse, she seemed to think that starting something with the MC was a means to that end (by making Chad jealous)! But that's just a rationalization. If Sage was only moving on because the relationship with Chad was dead she wouldn't have flipped out when he broke up with her.
Now, it's possible Sage will learn from this experience and be better able to handle a similar situation in the future; despite being a senior, she's still a young woman, after all. And her conversation with the MC at least points to some degree of introspection. But to date I see no sign that Sage recognizes the vast difference between the relationship in her mind and the relationship as it actually existed. Until that happens, I think there's a risk that Sage could eventually find herself in another relationship where rationalizing an affair would be easier than confronting the problems with her boyfriend head on.
(To be clear, I'm not saying Chad's treatment of Sage was okay; it absolutely was not. I'm just saying that, from everything we've seen, Sage ignored numerous indications of how disinterested he was in her.)