Redhat 9 is somewhat outdated and if it matters to you no longer supported by the company, though community support exists.
I personally don't recommend Fedora for production machines due to its basic nature as a beta/developement platform for RHE though lots use it successfully.
Debian is very stable. Woody, while it is a couple of versions behind many common packages is exceptionally stable and easily updated. Sarge is also fairly stable. Of your list either of these would be my choice. Slackware would be my overall choice since you are already familiar with it.