NetBSD has the widest support, continuing to run on most 68Ks and PCI Power Macs to this day (leaving out only the NuBus Power Macs which aren't really supported by much of anything anymore, sadly). However, OpenBSD works fine on New World Macs, and FreeBSD has a very mature 32-bit PowerPC port — or, should I say, soon will have had one, since starting in FreeBSD 15 (13.x is the current release), ARMv6, 32-bit Intel and 32-bit PowerPC support will likely be removed. No new 32-bit support will be added, including for RISC-V.
Even though I have a large number of NetBSD systems, I still like FreeBSD, and one of my remote "island" systems runs it. The differences between BSDs are more subtle than with Linux distributions, but you can still enjoy the different flavours that result, and I even ported a little FreeBSD code to the NetBSD kernel so I could support automatic restarts after a power failure on the G4 mini. The fact that the userland and kernel are better matched together probably makes the BSDs better desktop clients, too, especially since on big-endian we're already used to some packages just not building right, so we don't lose a whole lot by running it. (Usually those are the same packages that wouldn't build on anything but Linux anyway.)
This isn't the end for the G5, which should still be able to run the 64-bit version of FreeBSD, and OpenBSD hasn't voiced any firm plans to cut 32-bit loose. However, NetBSD supports the widest range of Macs, including Macs far older than any Power Mac, and frankly if you want to use a Un*x on a Power Mac and have reasonable confidence it will still be running on it for years to come, it's undeniably the one with the best track record.