Which file system can provide best space efficiency for backup storage?

In my working environment, we use some 2-8TB disks for backup storage. Most files are around 50-500MB. The files are written once, and rarely read. In this case, we care the space efficiency, rather than I/O performance.

I was using ext3/4, and I found that the space efficiency of ext3/4 is not so good. There are around 10% space are used by the meta-data of FS itself. Thus, I want to switch to a file system with better space efficiency. Which file system is the best choice for my scene? Thanks!