Q: Should I use NTFS or ReFS as the file system for my Hyper-V VHDX files?
A. At this time, do not use ReFS with Hyper-V. There are certain features of ReFS that just don't work with VHDX (such as integrity streams, which is a ReFS feature that writes any changes to new blocks on disk to remove any chance of data corruption). Thus, the guidance is to continue to use NTFS for live Hyper-V VHD/VHDX files.
If you have a library of VHDX files or a gold image collection that aren't used by running virtual machines (VMs), then they would be a good candidate to store on ReFS, with integrity streams enabled, for the highest level of resiliency and protection. I expect this will change in future versions of Windows and Hyper-V.