This article is from the Amiga FAQ, by Ignaz Kellerer with numerous contributions by others.
XFH does a good job. It operates as a handler and uses the XPK-libraries, so you have different compression modes (RAKE is a good choice) and possibly even more in the future. The only disadvantage is, that the size of a file is limited by RAM: Don't use it with less than 2MB of RAM.
XPKDisk by Olaf 'Rhialto' Seibert is another stacker program that takes advantage of the variety of XPK libraries available. Unlike other harddisk compressors it does not compress the files, but creates a pseudo partition and stores the whole tracks as compressed files on your harddisk. Its major advantage is that it does not limit the maximum file size, because it works similar to trackdisk.device and therefore does not need too much temporary storage.
But be careful if you want to use ReOrg on a comprimized partition: ReOrg should be set to leave enough memory for the xpkdisk buffers. And because of ReOrgs buffering, you can use few buffers for xpkdisk (perhaps as few as 2 or 3). Don't use ReOrg if you don't know the memory needs of XFH or XPKDisk.
Another possibility is EPU. It's shareware and should offer the same as XFH, but without limiting the file size.
Sources: Aminet, directory `util/pack', Fish disks 754 (XFH) and 858 (EPU)