I would have to say the hive. I don't think the darkness just created them from nothing. They had to come from somewhere, and I want to know about what and how they were before the Darkness. Also, although most of the hive seems mindless, especially the thralls (their name says it all), the wizards and acolytes seem to possess individual will. Do they, like their name suggests, worship the Darkness just as Guardians semi-worship the Traveler? And is it purely that they are in awe of the Darkness, or did it, like the Traveler with humans, do something to earn their devotion? I'm more curious about this race then the others- a space legion, time-robots, and militaristic nomad/invaders don't carry as much mystery as the hive.