Is Mac OS the most suitable OS for a beginner Data Scientist?
You will not find the most suitable OS for data science. It will come down to preference and what tools you will use. Data science is broad, what tools people work with will vary, some will have support in one OS and less in another.

All OSs are suitable and used by data scientists. Often you don't get to pick yourself. But very few things are exclusive to one OS, so it all works out. I would say you should pick the one you are most comfortable with. You want to spend more time learning data science and less time learning a new OS.