I would use T&G waferboard (OSB: Oriented Strand Board). I have seen a couple products that are used for under plywood systems for basement floors and they all use OSB. This is because no matter what you do, there will always be SOME amount of moisture and the OSB can expand and contract without warping or delaminating much better than any plywood (except SUPER expensive marine plywood). Don't forget your rigid foam insulation...
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