One of the *buntu netbook remixes - I wouldn't touch 'regular' Ubuntu with a long, shitty stick mainly because Unity is an absolute abomination.
I have Lubuntu installed on some of my lower spec (ie. less than 1Gb RAM) machines at work and it's more than adequate - lxde is a nice, lightweight window manager anyway. Xubuntu is more of the same (replace lxde with xfce4) but a bit heavier on resources, although not much.
Its been a long while since I actually had a netbook (a Dell Mini 10v) but if memory serves I was running Fedora on it, which is something else to consider.