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?
Debian is now the leading Linux distribution being used. Debian and Ubuntu have gained market share and have overtaken CentOS. More details can be found here - http://w3techs.com/technologies/details/os-linux/all/all