Choose your favorite metric: mindshare, running instances, development target. Free and low-end beat expensive proprietary.
"Least popular" is an odd claim. If you count Windows desktops, and then count all the Linux installations (desktop, servers, cloud, Android, other devices), I suspect Linux would prove to be most popular. Sure, add Windows servers to the contest, that's a rounding error. And what about VMs? How do you think Windows VMs stack up against Linux VMs in number?
"Least popular" is an odd claim. If you count Windows desktops, and then count all the Linux installations (desktop, servers, cloud, Android, other devices), I suspect Linux would prove to be most popular. Sure, add Windows servers to the contest, that's a rounding error. And what about VMs? How do you think Windows VMs stack up against Linux VMs in number?