Which filesystems are most space efficient
 

I'm considering use a different filesystem for the data drives of my NAS. These drives are for pure data (the root and home directories are elsewhere). For the most part, these drives are read from but not written to (my home movie and music collections, for example). The drives are almost full and for organizational reasons I'd prefer not to split them across two drives at the moment. I'm not running raid. The box runs 24/7, though I do spin the drives down when not in use. Windows compatibility is not an issue as I never take the drives out of the box. Also, most of the files are already compressed, so I'm not really considering filesystems that feature automatic compression.

At present I'm using ext4 for these filesystems but I read somewhere that ext4 is somewhat wasteful with the disk space. Also, since there's very little writing to disk I'm not sure I benefit from the journalling of ext4. On the other hand, if it's a very small amount of space used for journalling, perhaps it is still worth it.

Anyway, I'd like to not waste space on features I don't need. I'm having a hard time getting information about how efficient the various filesystems are in terms of space usage. Is there a ranking, or perhaps someone can point out their top few suggestions for my case?

Here are some older benchmarks that may help:
http://linuxgazette.net/122/piszcz.html
http://www.debian-administration.org/articles/388

I think XFS and JFS are your best bets. Reiserfs is not maintained, so ignore it. I could not find any ext4 benchmarks for space efficiency, so I can only assume that it is like its predecessors.

ZFS supports compression. IDK how mature and stable it is though.

Great answers, guys. Based on this I guess I'll format my new drive to XFS.

Thanks a lot!