If permissions need to be preserved you should use ext where ext4 should be preferred.
If the files are personal and you do not need to care about permissions you could use a windows file system. FAT32 has limits in filesize so I would advice against it. NTFS should be preferred.
I myself would always use the same file system as what you use for the originals.
Depends on what you want to backup.
I myself would always use the same file system as what you use for the originals.