Which edition of Windows Server 2012 R2 should you choose for VMs?
Today, that is no longer the case as Microsoft removed the technical differentiators between the Standard and Data Center editions of with the release of Windows Server 2012. Now, the only differentiator revolves around licensing, and that only matters at the host level.
With a Data Center edition of Windows Server 2012 R2 acting as a Hyper-V host or for which the license if assigned to a vSphere host, all of the virtual machines that run on the host are already licensed. The organization does not need to individually license each individual virtual machine. The Standard edition only allows running one more OSE (Operating System Environment) beyond the host, so you can basically run the host plus one virtual machine.
As always, make sure you check with your Microsoft software reseller before committing to a purchase as Microsoft is constantly changing the licensing rules.