I personally run truenas on a standalone system to act as my NAS network wide. It never goes offline and is up near 24/7 except when I need to pull a dead drive.
Unraid is my go to right now for self hosting as its learning curve for docker containers is fairly easy. I find I reboot the system from time to time so its not something I use for a daily NAS solution.
Proxmox I run as well on a standalone system. This is my go to for VM instances. Really easy to spin up any OS I would need for any purpose. I run things like home assistant for example on this machine. And its uptime is 24/7.
Each operating system has its advantages, and all three could potentially do the same things. Though I do find a containered approche prevents long periods of downtime if one system goes offline.