BTRFS in FS-managed RAID configuration for automatic self-healing and snapshots for instant automated backups (though I keep a traditional backup too for protection against bugs and user error).
Storage is cheap compared to how much I value my data. BTRFS has very good support on Linux, integration with some backup tools, and I really want to use a FS that has full data checksums to make sure the data stays correct at rest. I like that I don’t have to use equal sized drives and can use whatever I have available, though I would appreciate a better read distribution model rather than the current where it just chooses a random drive to read from when multiple copies are available.
Disadvantages include difficulty accessing from Windows in my experience, less than stellar performance on HDDs, not very space efficient for small files systems because of the bulky metadata, and some uncommon RAID types don’t work correctly and will eat your data. I also don’t recommend it for use over USB because many such devices don’t correctly implement sync, and this is very important to stay on the correct transaction number and prevent file system inconsistencies. If I had to boot from USB, I wouldn’t pick BTRFS.
I don’t think exFAT or FAT32 offer POSIX permissions. I’m not sure if you could have a root file system there.