For most systems, ext4 because it seems stable and uncomplicated.
For my NAS and big data, ZFS. People whose opinions I trust recommend it, and to the best of my technical ability to evaluate said things, the claims make sense and seem to be extremely beneficial against the threats I perceive to my data.
I have been trolling people for years with passionate wat-too-serious arguments that the rainbow is ROY G BAP for blue and purple.