Which? Apple won't adopt anything GPL, so BTRFS, XFS, JFS, etc--basically every Linux project is right out. It was almost ZFS, years ago, but Apple backed out when Oracle bought Sun. What out there is modern, good, and under a compatible license? HAMMER?
IIRC, they hired Dominic Giampaolo to work on the file system for a while -- he was responsible for adding journaling to HFS+ and started Spotlight, which has a lot in common with BeFS's attributes. He apparently worked on a new file system for them but it never shipped.