Debian.
My reasoning behind this is that from all the operating systems I've used, *BSD, RHEL-Based OS, SUSE-based OS, etc. I've seen Debian be one of the easiest and straight-forward server OS to work with. I wouldn't pick Ubuntu because I feel that Ubuntu is overcluttered and at times just makes things more difficult to do while they claim it's "making it easier". RHEL to me just gets confusing. I started out working with CentOS and just did not enjoy my experience at all. Now I'm not saying I'm horrible with CentOS, but I do prefer Debian over CentOS anyday.
If this was desktop though, I'd support Crunchbang because of how easy it is to work with it (especially the shortcut keys out of the box and just the UI experience I get). Fortunately for me Crunchbang is debian-based (any debian software works with Crunchbang), so if I support Debian I'd indirectly be support Crunchbang regardless.