Let's consider the file systems' applicability to a RAM disk by attributes:

Safety (Journaling)

Conceptual Notes

If you are meaning to cause your RAM disk to eventually persist back to disk, you would need your RAM disk software to periodically write any changes back to a hard disk backup file, so that if you reboot, you don't lose (too much) data.

If you were to cause the RAM disk backup file to be constantly overwritten, you'd be totally defeating the purpose of the RAM disk, because that would be the same as just writing the files straight to the HDD in the first place. So in other words, any data stored in the RAM disk should be considered expendable (if your system turned off right now, the data would be gone) for any writes to the RAM disk which occur between the synchronization point(s) to the persistent media (your hard disk).

If you don't want your RAM disk to be file-backed at all and can afford to lose the data whenever, then disregard data safety entirely.

NTFS: It has a feature called "journaling", which basically means that the file system is always in a consistent state; it's never left in a state where a write command is only "half done", because every write gets staged on the disk first (either in the metadata journal, the data journal, or both) and then finally committed. This is great for non-volatile media like hard disks and SSDs, but it is useless for RAM disks. The main thing that a journal tries to prevent is data loss during a sudden loss of power to the PC, or a software bug so severe that it causes the system to crash without being able to synchronize the file system (e.g. a Blue Screen of Death). But since the journal and the file system's data is in RAM, no amount of data journaling can prevent data loss! So the journal is just wasted space and I/O operations.

FAT32: Does not have a journal.

FAT16: Does not have a journal.