Strange, no ext4. I see another reply suggests XFS and Btrfs. I disagree with XFS, it is mostly meant for storing large files, whereas during common use with user files, and config, you get a lot of small files.

You could use Btrfs, while normally it is pretty stable, and has some advanced features like snapshots, when it does break, the recovery tools are really unfriendly for normal users. Since external drives are not usually very high reliability, I would be slightly hesitant. IMO, ext4 is the tried and tested, reliable choice.