I think the Gold Dragon. I like their scholarly interests and their mix of ability that makes them great in a variety of roles, from combat to support to mentoring to employer. They can work with the PCs as scholars, as mentors, as non-lethal opponents.
Plus their shape-shifting abilities makes them extremely useful for infiltration. I was able to write the story of a family of them in Living Greyhawk and really thought it was a fun addition.