It depends on your DE preference, really. Here's what I think:
[Ubuntu] Gnome: Most stable, not too resource intensive.
[Kubuntu] KDE: More resource intensive, less stable, but much more attractive graphically.
[Lubuntu] LXDE: Stable, very low resource usage.
[Xubuntu] XFCE: Less stable (from my experiences), very low resource usage, very attractive, minimalistic look.
If you're a newbie, go for Ubuntu.
[Ubuntu] Gnome: Most stable, not too resource intensive.
[Kubuntu] KDE: More resource intensive, less stable, but much more attractive graphically.
[Lubuntu] LXDE: Stable, very low resource usage.
[Xubuntu] XFCE: Less stable (from my experiences), very low resource usage, very attractive, minimalistic look.
If you're a newbie, go for Ubuntu.