Flavor of Linux which is ONLY command line (and super light-weight)?

To be particularly minimal, you could roll your own. Fair warning: this is an endeavor of many hours, but you'll better appreciate what goes into getting your OS pieced together.

Linux From Scratch is a very actively maintained step-by-step guide on just that: http://www.linuxfromscratch.org/

Red Hat or Debian are probably what your looking for. You could also install Ubuntu as command line only.