I'd say, Gentoo. LFS may be a little laborious, and at the end you'll not have the desktop you're used to.
Gentoo is how I learned it (after failing with BSD), it's excellent if you have "medium" to "much" experience with Windows. Some DOS experience, preferably with the 'edit' (DOS editor) command too, is a real advantage btw!
Gentoo also offers ways of trimming it down (if you disable 'doc' UseFlag, meaning no documentation, and disabling some others). Also, 6GB means no KDE and Gnome. You might want to opt for LXDE instead, it resembles Windows a bit and is leightweight. I'd recommend Opera instead of Firefox to save some extra space, and certainly AbiWord and Gnumeric insteaf of OpenOffice. Use a tiny swapfile as well, it saves some extra space.
DSL, Slackware and TinyCore are great distro's, but you will not learn very much, as the 'why' is missing in the manuals (though Slackware gets you almost there). Automatic dependency resolution is also something you'll almost definitely want, hence Gentoo.
Gentoo has pretty good documentation, and step by step guides.
Here's the handbook.
Google for "Gentoo nano basics guide" to learn how to use Gentoo's standard text-editor.
If you need help with Gentoo, let me know!