Depends. Slower desktop machines XFS.
Standard desktop XFS, if it has a smaller SSD, Btrfs.
Home server ext4/XFS + ZFS. Generic servers at work ext4/XFS, backup/storage servers ZFS.
Database server, experiment with ZFS with compression enabled - ratio 2:1, but encountered problems (probably a bad HBA model), standard ext4/XFS.
Hosts with virtualization, small server - XFS, big server - ZFS (technically a ZVOL).