Answer 1

I think you cannot tell, which os is the 'best', which one you choose depends on your personal needs.

For example someone who like to design stuff (and for whom money does not matter) will most likely choose macOS. If you want a secure, stabile and extensible system a linux distribution mostly is the choice (for servers also). Besides that, most distributions are free of charge. The benefits of windows are, that its easy to use and offers the highest amount of software.

Telling about mobileOs i would say choose what you like the most^^