Gentoo.
Why? One thing is portage, the package management. I used to use Debian before and just loved apt and its powers, and portage is quite similar. The reason I chose Gentoo over Debian is the support (forums, wiki, homepage) and the fact that many programs are in portage that aren't in Debian (many things not compliant to the DFSGL like mplayer etc...). And the fact that you can keep a system (quite) up to date without having to install relatively untested software (like in Debian unstable).