Really, they're all equally customizable. The question is, how much of that customization is required? If you go with Arch or Linux from Scratch you'll be obviously be choosing everything yourself; Gentoo, Debian, and Slackware are more traditional in that sense; but even the slickest distros don't stop you from changing anything. So, I'd pick (K)ubuntu, Fedora, or SuSE but dump them if you find they're doing too much for you.
Which brings me to another point: with the abundance of free distributions, why convince yourself that your choice is permanent? I've been on Kubuntu for a year, but I've also used Ubuntu, Fedora, SuSE, and Arch with KDEMod. It does help to install /home on a separate partition or hard drive, though.
Edit: I see from your other question that you're already using Arch, which is interesting because in my experience it was rather smooth after the big installation and configuration hump. Have you used KDEMod? A higher-level distro? Or maybe this more of a poll question.