If you run Redhat, I would advise the most recent; i.e., Red Hat Enterprise Linux 5, since they do not add any new features and only correct errors. CentOS is the same as Red Hat, but you probably get better support from Red Hat if you need it -- though you pay for it.
The other thing to take into consideration is the number of vendors who release drivers for linux distros. Typically, RHEL and SLES get the quickest release of drivers for things like network cards, storage cards, etc...