One word: Gentoo
Seconded. I started out with Slackware, used RedHat for ages, spent about two days on Debian fighting a losing battle with its package management, and ended up building a completely custom distro just before all those neat "build a completely custom distro" kits came out -- LFS, Sourceror, Gentoo. From what I've heard, Gentoo is the best of the three, especially for starting out with. I reckon if it had come out only months earlier it'd probably be on all my machines now.

Peter