An Operating System🔗 manages the hardware on your server. It also determines what kind of software you can run on the server. You must strongly consider your needs as some operating systems will incur more resource overhead than you may need.
FreeBSD is an open-source UNIX operating system. It is the most barebones of the lot, but has the potential for the best performance. Furthermore, it is exclusively command-line based, meaning there is a learning curve if you are not comfortable with the terminal.
Unraid is an entirely paid NAS software with the allowance of a 30-day free trial. It is special in that you can deploy a JBOD🔗 with ease. Due to it being XFS-based, it supports parity of up to 30 disks in one array. Unlike TrueNAS, you do not have to worry about matching the hard drive models.
Ubuntu Server is a server-oriented version of the popular Linux open-source operating system Ubuntu. It includes packages to get you serving quickly, such as Apache Web Server and SSH. Ubuntu server also has the option to install without a GUI, resulting in less operating system overhead.
Synology is primarily a NAS hardware and software bundle. This means you can buy a premade NAS box and throw disks in it and you are off to the races. Synology does, however, offer a wide-array of hardware options to meet your needs.
ProxMox is an open-source baremetal hypervisor alternative to ESXI, which is a prioprietary paid software by VMware technologies. It allows you to run many virtual machines on one machine, in which you can put a NAS software on top of.
TrueNas is a merged project between Community Edition FreeNAS and Enterprise-grade TrueNAS. It allows a free usage with paid options allowing for more disks and clusters. TrueNAS has superior performance over something like Unraid due to its ZFS-based file system. It is less flexible than Unraid in that it is best practice to use the same exact disks for a volume.