Have you considered
Arch Linux? It's sort of philosophically aligned with Gentoo and Slackware in that it starts as minimalist install and leaves it up to you to add only what you want (and assumes that you know what you're doing).
Unlike Gentoo, it's a binary distro (i686 optimized ... don't bother if you have antique hardware). The package manager (Pacman) is quite good and it's
very easy to create your own packages (much easier than Debian). The pre-packaged stuff is pretty current (gcc 4.2.0, kernel 2.6.21, xorg 7.0, gnome 2.18, kde 3.5.7, xfce 4.4, etc.) and there is an active community maintaining even more bleeding edge versions.
I'm still partial to Ubuntu for boxes that I want to just work. But if you want to tinker, Arch is a great choice.