I use Debian or Ubuntu (which is based on Debian anyway) since I prefer apt-get over the rpm tools for managing the system. But really, that's more preference than anything else. I've used Fedora and Red Hat before that for a long time.

They're all fairly similar and pretty much equally configurable as far as servers go. Just choose one, learn the tools you need, automate with shell scripts, and away you go :)

I find that the key for manageability is scripts. I have a repeatable mostly-automatic process for building new servers from a base install that I've been using and tweaking since my first server, now it handles everything from Apache and MySQL to iptables, backups over rsync+ssh, etc.