We are using Debian as our main server's operating system. This includes services such as SMB, commercial antivirus, GLPI, MySQL database, and, in the future, OpenLDAP. Also, we are deploying desktop machines with LXDE as desktop environment. We already have four test machines, but next year (2015), we are planning a deployment of 100 computers, which is one third of our desktop machines.
We chose Debian because of its great stability and long term support. We also use Debian because it has received great reviews as a server solution.