My first choice in material for floor repair would be marine plywood.
But who can afford the stuff?
So I would use MDO.
MDO is awesome suff. Made to stand up to the elements and seems to be stiffer that like thickness ACX.
Most of the ACX I see is just bearly good enough- voids, warped, plug repaired.
The MDO is high quality, rock solid, dead flat, stiff and reasonably priced compared to ACX.
There is a reason freeway signs, billboards and concrete forms are made from it.