For me, it was kubuntu. Back in late 2005 or early 2006. The reason? They were always pretty good at shipping the latest KDE. I had grown tired of hoping someone would compile a new version for my preferred distro.

So kubuntu it was, and has been ever since. I'm currently looking into whether I should change to something else - as I've started growing tired of Ubuntu/Kubuntu after some 20 odd years.