I voted for slackware, because, in my, and many others' opinions, it is the best for learning how everything works.
Sure, no hand-holding in this distro, but thats one of the 'features' that forces you to get your hands dirty.
And getting your hands dirty is the only way to learn.
One thing I like I like about Slackware is that it has a very clean design. The config files aren't stored in some weird place, they are where they are supposed to be, or where you *want* them to be. Another thing I like about it is the fact that its complete *not bloated*.
Just one thing to remember with Slack :
**********READ ALL YOU CAN*************
Good luck at choosing a distro!
Refriedbean
PS, Other distro's that I recommend are Gentoo, Debian, and if you feel adventurous LinuxFromScratch.