Row 20 is probably the best on the MAX.
Row 31 left or right side (not centre) has the most space on the 788/789, although as mentioned above, the seats are slightly narrower (I'm okay in them, but I'm not very wide).
The middles are often empty, so I've seen couples book the window and aisle and hope that no one takes the middle. If someone is in the middle, that person is usually happy to switch.