Debian for sure. Debian Stable even, or Testing at most. It's to me the most professional and stable system, and it has a good spirit, not as commercial as Fedora or SuSE, and tried and tested.
Nowadays I'm starting to use BSD as much as possible for servers, because of the stability and security (OpenBSD as internet gateway/router/firewall/etc), but as for Linux, I'd go for Debian.
I'm using Ubuntu now for desktop, just for fun and newer software, but I have often gone back to Debian when I got tired of the system. Oh, and ofcourse Ubuntu is Debian based..)
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