I voted Ubuntu, to be specific I use Lubuntu. I had a 2006 laptop with XP on it which I wanted to run Linux. Lubuntu was the only distro that ran without any issues. Most posts I read about installing Linux on an old PC recommended Xfce distros but I found Lxde much less resource intensive. Anyway after my old laptop ran fine I installed Lubuntu on my work PC and have been happy with the performance.