I had a similar discussion the other day and to me, it's always seemed logically like it should be Abigail for a few distinct reasons:
1. She's someone who explores and is out and about most of the day, even exploring our farm and reminiscing on the fun she had doing so prior to our moving to the valley
2. She wants to do stuff we do like fighting and entering the mines, some people fixate on that the ideal spouse should be someone who's suited to the 'farm spouse' sort of role, but we hardly fit that ourselves, doing fishing, mining, and combat equally if not more than mining. This also means we're suitable as a pair to do these activities together, or at least hang out in the valley together, such as her playing the flute at the mountains while we fish.
3. She didn't come to the valley with a developed goal/to escape something, and doesn't have set aspirations that'll lead her out of the valley. Unlike someone like Leah who came to the valley to escape the city life and has her own settled life, or someone like Sebastian who wishes to explore the big city life in his future, Abigail's goals are far less concrete, and most of them are rather valley-centered, things like exploring and combat being possible within the valley. She also doesn't have her own house which means we're not really taking much autonomy from her in that regard.
My only critique to this argument is that we don't really actually see it happen, as much as its hinted at, we have no way of letting her follow us into the mines, and not even any dialogue pointing towards her doing this with us, which would be reassuring to see. If that was ever added in basically any capacity, I'd be completely sure she's the most suited towards the SDV farmer (farmer/fighter/miner) life alongside us as players.