Personally, I prefer Xubuntu, lightweight distro that runs good on anything no matter how new or how slow and ancient. Plus I can browse the internet, send email, chat on IRC and do everything you would do with a computer while the OS is installing.
However, if the computer at least 1GHz w/ 768 RAM and intended for someone other than me, I would install Ubuntu then change the window manager from Unity to KDE. Lots of nice eyecandy. (Or for a quick install, just use Kubuntu, but I personally don't think this is as good of an option.) In a few cases I have seen Kubuntu run better on an old machine than Xubuntu.
All in all I have been exceedingly happy with Debian and Ubuntu derivatives.