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?