> the world's most popular OS

Wikipedia claims that Android "has the largest installed base of any operating system in the world", if you're going to measure popularity that way.

(Of course it's hard to know how to define an OS. Is Android a kind of Linux? Are the various things called "Windows" or "MacOS" to be regarded as different versions of the same OS just because marketing people decided to use the same name? If not, how much similarity in code or design is required?)