It's very likely the most popular OS shipping today on desktops, laptops and servers. All Intel chipsets post-2015 are running MINIX 3 internally in the Intel Management Engine.
It's not the most popular overall because the largest number of CPU chips in the world are ARM (phones and other devices) which don't have embedded MINIX.
You could argue that the most popular shipping OS is the BIOS, or hard drive microcode.
