Which distribution does have best market share rates?
Is it still CentOS?
I'm guessing Ubuntu server has already overtaken others. But I'm not sure.
Is there a reliable document about this?
CentOS is still the leading linux distribution being used. Ubuntu has gained market share but still has a lot of catching up to do. More details can be found here - http://w3techs.com/technologies/details/os-linux/all/all