"Root FS" alone is quite non-descriptive term since it's unclear will you use another mountpoints and what/where are they.
So, in common — quite space efficient is Btrfs with
compress=lzo (or even
compress-force=), and it was mine recent choice for root FS, but mine root FS is read-
onlymostly since I've separate FSes for, say, /home (EXT4) and torrent storage (which used to be XFS, but now is also Btrfs, BTW).
In despite of what your choice would be I strongly encourage you to use LVM-2 since it'd give you flexibility with all those FSes you come along. ;-)