Ext4 for everything when possible, because its reliable and proven. I’m looking towards Btrfs for my next system drive, as it is mature now and has good features. But I would use Ext4 for everything else still. For interoperability that doesn’t understand Ext4 it would be NTFS when supported, otherwise fallback to FAT32.
That’s the entirety of my knowledge and what I use when I have to format it myself. :D