To me, X feels like it's the more modern sensibilities of Square desperately trying to make something that still meets the standards of the classics. It doesn't feel quite right, isn't able to land that attempt, feels very shallow and emotionally manipulative in comparison, and I personally find it bad overall, but at least the attempt at beauty is there.
XIII and XV are just high-production-value pure trash, devoid of anything resembling real value or motivation-- I'd go as far as to say that I don't even respect them anymore than some stupid thing that David Cage makes, personally. They're arguably better than FFII (which is borderline unplayable) and that's about it, IMO.
For me, it goes
Very Good: IV, VI, VII, Tactics
Good: IX, V, VIIR
Hit or Miss: VIII, XII
Bad: X, III, I
Terrible: XIII, XV, II, X-2