CentOS is a better server for Linux Distribution
Debian is also another good choice well tested before release thats why there is only Debian 4.0 Stable now
What make a good server OS?
Stability Compatibility High performance
High Performance: Debian and CentOS makes good choice because they install very little programs by default so less programs are running which give them higher performance
Compatibility: almost all Linux Distro have SAMBA support which makes them compatible with Windows Client
Stability: CentOS is based on Head Hat linux Enterprise Edition whihc is actually a commercially stable server OS
Debian stable as i mention earlier has a very stringent release cycle hence it is robust robust
Besides Linux Distribution Another highly acclaim Server OS is FreeBSD 7.0 Stable it is slightly different from Linux so you might have to learn another UNIX system but again it is reputable many web companies used them including yahoo
and FreeBSD servers are known to have very low down time
Some run for years without requiring any reboot
It also supports SAMBA etc
All the above OS I mentioned above also has good security supporting OpenSSH etc