Maintain Windows Server from time to time. As to your question, depends what you want to do, as a rule though the latest version tends to be best. Fully supported etc. Later versions may require use of Powershell for configuration and there's an option to install with no GUI. Handy thing in later versions is Hyper-V so you could virtualise earlier versions and test them.
As @Him Her said, it depends what you really want it for.
If your PC is up to it you can use Hyper-V on Windows 10 to install guest VM's of Windows Server right up to 2019 for testing the waters.
Persoanlly I like to use VMware Workstation for virtualization because I've used it for years. It's a bit easier to use I think if you're new to it all. It's not free however but is very easy to get a license key if you're prepared to 'borrow it' ahem, so to speak.
VirtualBox is also a popular hypervisor you can use. It used to be free and probably still is but I haven't used it in years.
VirtualBox is still free, I'm using it on the Linux boxes. VMware is on some sites too (I've been 'unretired' to help the eldest out in his support business).
Hyper-V is used internally because it's a Microsoft Partner business but I'm also running labs on it. Nested VMs, multiple VMs on VMs on Hyper-V to simulate customer environments. You can probably do that on the other hypervisors too but Hyper-V was the first one I got working!