Posted a bit late last night and didn't say everything I wanted to. If I were installing a new distribution for myself and Arch were out of the running, I'd go with Gentoo or Slackware. Part of the fun of Linux for me is learning new things, and I learned enough from my first install of Gentoo to where this time I could probably skip past the bad choices I made in the beginning that eventually convinced me to jump ship. And Slack, I've just always loved the way it's put together.
But if I were to install a distro on another person's PC, I'd have to go with Debian or CentOS. Arch is out of the running because blindly calling pacman -Syu (the Arch equivalent of apt-get update && apt-get upgrade) can break things, sometimes seriously. That's the downside of running a rolling-release distro--sometimes big changes in something like Gnome will break something somewhere else, and if I'm not 100% sure the user is going to read the upgrade notes BEFORE upgrading, I'd really rather not deal with his phone calls.