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.
It depends on your accounting, so I like to leave the door open for debate. The Linux kernel does have a good case if your metric is total installs. If you go by market share, Windows might have had the highest proportion ever in the 1990s (not sure, but it was dominant).
If you count by "dollars went to OS vendor for the OS explicitly" then yes. Otherwise Android phones alone is like Apple+Ms combined in terms of installs.
That's exactly my issue with going by market share. By installations, desktop Linux is massively successful compared to where it was in the 90s even though its share is low.
