CentOS is based on RedHat Enterprise, which has been considered the standard when talking enterprise Linux.
Debian provides access to a large number of packages. Possibly the largest repository of applications for any Linux distribution.
openSUSE is based on SUSE Enterprise Linux. It is a well documented distribution, with excellent community support.
Ubuntu Server allows for easy deployment of application from one of the largest repositories available in any Linux distribution.
Windows Server 2008 R2 builds on the award-winning foundation of Windows Server 2008.