I voted for Slackware, because in the real world that's what I use on older, slower systems. If the system is moderately slow, then I'll run kde with the eye-candy way down, but if it's really slow, I'll use xfce or no gui at all.

However, that being said, I have also used Damn Small. It is really resource friendly and is definitely worth looking at for someone with a really old PC like a 486 or something under 200MHz.