I’m using mine very lightly, but I’ve got the Plex server running on the Synology NAS. I’m doing any transcoding on the gaming desktop and storing the output on the NAS.
I would say Synology NAS or Ubuntu. Both have worked well for me.
I originally set up Plex on FreeNAS, but their plugin wasn't very good (too many crashes), so I went to Ubuntu. That was few years ago, FreeNAS could be better now. Once I got the Synology NAS I tried it, but I couldn't see any performance difference vs. using Ubuntu+NAS, so I stuck with that. The Linux Plex server just runs, and I never have to touch it.