1) The Portalus Group, Brooklyn, NY, USA
We use Debian for our internal infrastructure and for consultation deployments. We consistently choose Debian over other operation systems primarily due to the open ended support, quick security updates, excellent package management, documentation, and the accessible community.
Our network started with 2 servers running Debian and is now at 10 servers with plans for new local hardware of 20 servers for which 17 are slatted for Debian installations. We use Apache2, OpenLDAP, Samba, Postfix, Dovecot, Digikam, and mysql.
Debian has been reliable and solid from the first day of installation.