We are using Debian as our server's operating system. This includes services such as: Apache, Postfix + Spamassassin + Amavis, PostgreSQL database + PostGIS, MySQL database, Docker, Geoserver, etc. Also, we have installed Debian on some desktop machines with graphical desktop environment.
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. We also use Debian because it's free and open.